Add merge request templates for key actions

.gitlab/merge_request_templates/*.md:
Add templates for merge requests that add or remove new packager or main
keys.

.gitlab/merge_request_templates/New Main Key.md

+<!--
+This template is used when a new main PGP public key needs to be added to the
+distribution's keyring.
+It is used by users with a valid packager key after all steps in an
+accompanying issue (opened with the template "New Main Key") have been
+fulfilled.
+-->
+/assign_reviewer @allan @anthraxx @bluewind @dvzrv @pierre
+/label ~"new main key"
+/title Add main key of <!-- MODIFY: Add the main key holder's username -->
+<!--
+Please do not remove the above quick actions, which automatically label the
+issue and assign relevant users.
+-->
+
+# Add a new main key
+
+## Details
+
+- Username: <!-- MODIFY: Add the @-prefixed username -->
+- PGP key ID: <!-- MODIFY: Add the "long format" key ID of the PGP public key here -->
+
+Closes <!-- MODIFY: Add #-prefixed issue number, that will be closed by merging this merge request -->
+
+## Checks
+
+### Keyring maintainer
+
+- [ ] All steps in the accompanying ticket are fulfilled.
+
+### Main key holders
+
+- [ ] The public key has been validated according to the [best
+  practices](https://gitlab.archlinux.org/archlinux/archlinux-keyring/-/wikis/best-practices#validating-a-key-pair)

.gitlab/merge_request_templates/New Packager Key.md

+<!--
+This template is used when a new packager PGP public key needs to be added to
+the distribution's keyring.
+It is either used by the sponsor of a new packager or by an existing packager
+when adding a new key for themself after all steps in an accompanying issue
+(opened with the template "New Packager Key") have been fulfilled..
+-->
+/assign_reviewer @allan @anthraxx @bluewind @dvzrv @pierre
+/label ~"new packager key"
+/title Add packager key of <!-- MODIFY: Add the packager key holder's username -->
+<!--
+Please do not remove the above quick actions, which automatically label the
+issue and assign relevant users.
+-->
+
+# Add a new packager key
+
+## Details
+
+- Username: <!-- MODIFY: Add the @-prefixed username -->
+- PGP key ID: <!-- MODIFY: Add the "long format" key ID of the PGP public key here -->
+
+Closes <!-- MODIFY: Add #-prefixed issue number, that will be closed by merging this merge request -->
+
+## Checks
+
+- [ ] All steps in the accompanying ticket are fulfilled.
+
+### Main key holders
+
+- [ ] The public key has been validated according to the [best
+  practices](https://gitlab.archlinux.org/archlinux/archlinux-keyring/-/wikis/best-practices#validating-a-key-pair)

.gitlab/merge_request_templates/Remove Main Key.md

+<!--
+This template is used when an existing main PGP public key needs to be removed
+from the distribution's keyring.
+It is used by users with a valid main key after all steps in an accompanying
+issue (opened with the template "Remove Main Key") have been fulfilled.
+-->
+/assign_reviewer @allan @anthraxx @bluewind @dvzrv @pierre
+/label ~"remove main key"
+/title Remove main key of <!-- MODIFY: Add the main key holder's username -->
+<!--
+Please do not remove the above quick actions, which automatically label the
+issue and assign relevant users.
+-->
+
+# Remove a main key
+
+## Details
+
+- Username: <!-- MODIFY: Add the @-prefixed username -->
+- PGP key ID: <!-- MODIFY: Add the "long format" key ID of the PGP public key here -->
+
+Closes <!-- MODIFY: Add #-prefixed issue number, that will be closed by merging this merge request -->
+
+## Checks
+
+### Keyring maintainer
+
+- [ ] There are more than or equal to three valid main keys remaining after
+  removal of this key.
+- [ ] All packagers have at least three valid main key signatures for their
+  packager key after removal of this key.
+- [ ] The key has been revoked.

.gitlab/merge_request_templates/Remove Packager Key.md

+<!--
+This template is used when an existing packager PGP public key needs to be
+removed from the distribution's keyring.
+It is used by users with a valid main key or a valid packager key after all
+steps in an accompanying issue (opened with the template "Remove Packager Key")
+have been fulfilled.
+-->
+/assign_reviewer @allan @anthraxx @bluewind @dvzrv @pierre
+/label ~"remove packager key"
+/title Remove packager key of <!-- MODIFY: Add the packager's username -->
+<!--
+Please do not remove the above quick actions, which automatically label the
+issue and assign relevant users as reviewers.
+-->
+
+# Remove a packager key
+
+## Details
+
+- Username: <!-- MODIFY: Add the @-prefixed username -->
+- PGP key ID: <!-- MODIFY: Add the "long format" key ID of the PGP public key here -->
+
+Closes <!-- MODIFY: Add #-prefixed issue number, that will be closed by merging this merge request -->
+
+## Checks
+
+### Keyring maintainer
+
+- [ ] There are no packages left in any of the official repositories, that are
+  signed by the key which is about to be removed.
+- [ ] All main key holders have revoked their signatures for the key.