`CommitGPGSignature` was originally made to store information about a
commit's GPG signature. Nowadays, it is used to store information about
SSH signatures too, and not just commit signatures, but tag signatures
too.
As such, rename it to `ObjectSignature`, because that describes what it
does a whole lot better.
Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>
