With the support for network.wireguard.* credentials[1] in systemd
v256[2], we can now easily avoid storing the credentials centrally in
our ansible vault, which is preferable as it makes the private keys less
exposed.

All the keys have been rotated and the new private keys are only stored
on the servers.

[1] https://github.com/systemd/systemd/pull/30826
[2] https://github.com/systemd/systemd/releases/tag/v256