Fix: Thread safe GPGME tmp directory

What does this MR do?

Makes all Gitlab::Gpg operations thread safe.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

As GPGME stores the home directory on the class (or module) multiple threads using different temporary gpg directories unset the other's directory and end up using the wrong one. This leads to incorrect gpg operations, e.g. wrong signature validations.

The development of this MR is sponsored by @siemens (/cc @bufferoverflow).

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/35986