Add terminating newline to /:username.keys and /:username.gpg

What does this MR do and why?

This change makes the output compliant with POSIX 3.206 (which defines a line as a sequence of non-newline characters plus a terminating newline character) and RFC 4880 section 6.2 (which specifies that the terminating newline is to be considered part of the ASCII armored key), respectively, for improved interoperability, e.g. when adding to authorized_keys files.

MR acceptance checklist

Note: I have not actually tested this change, since I don't have a working gitlab instance set up and the change seemed entirely trivial :)