Ensure users can't create environments with leading or trailing slashes (Fixes #39885)

What does this MR do?

Fixes #39885 (closed)

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

Figure out how to handle existing environments that are now invalid. Is this going to be an issue for people updating their own gitlab instances?

Why was this MR needed?

To ensure that future environments created with a leading slash do not break the environments page.

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
• [NA] Internationalization required/considered

What are the relevant issue numbers?

#39885 (closed)