Return 404 if deploy key cannot be found in Projects::DeployKeysController#disable

What does this MR do?

If a deploy key cannot be found, return 404 instead of calling destroy on nil.

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

I don't think that this path needs test. However, Projects::DeployKeysController#disable was not covered by any tests so I added a test for the happy path.

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • [ ] Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #29633 (closed)