Ensure validation messages are shown within the milestone form

What does this MR do?

Fixes a bug where upon entering data to create an invalid milestone via the Web UI or the API would raise an exception rather than render the validation messages

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

Please check specs introduced to avoid a regression

Why was this MR needed?

We'd rather render the validation messages to inform the user than raise an exception and result in a 422 error being displayed

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• 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 you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #22033 (closed)