Refactor Gpg::Commit to delegate signature methods

What does this MR do and why?

• In Render verification status for gpg signed tags (!200868 - merged) we extracted a class from Gpg::Commit.
  • This change is to refactor Gpg::Commit by delegating to that extracted class.

Note: The diff is probably easier to read if Show whitespace changes is unchecked

How to set up and validate locally

1. Create a gpg key (https://docs.gitlab.com/user/project/repository/signed_commits/gpg/#create-a-gpg-key)
2. Create a project
3. Enable the gpg_commit_delegate_to_signature feature flag
4. Create signed commits and push them to the project as the user with the gpg key
5. Check that the commits are verified as expected.

MR acceptance checklist

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Related to #19260