Fix: GPG tmp dir removal race condition

What does this MR do?

Fixes the race condition happening when removing the GPG's tmp directory.

Ignores any errors when removing the tmp directory, as we may run into a race condition: The gpg-agent agent process may clean up some files as well while FileUtils.remove_entry is iterating the directory and removing all its contained files and directories recursively, which could raise an error.

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

Why was this MR needed?

This results in Internal Server Errors (500).

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/36998