mailman: update mkinitcpio URL in arch-projects mailing list description

mkinitcpio moved from GitHub to gitlab.archlinux.org. See #474 (closed).

/cc @foxboron

added 6 commits

• 723f84f2...046efd7f - 5 commits from branch archlinux:master
• 62667cd9 - mailman: update mkinitcpio URL in arch-projects mailing list description

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added 1 commit

• ee84eb06 - mailman: update mkinitcpio URL in arch-projects mailing list description

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mentioned in commit ef1f5c81

merged