Change GraphQL milestone property to be a String

What does this MR do?

This MR changes the GraphQL milestone property, used when deprecating fields, to be a String.

An issue with using Floats to define the milestone of a deprecated field surfaced with the milestone 12.10. As a Float, this becomes "12.1" !28560 (comment 316709552).

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done