Make milestone a string in metrics schema

What does this MR do?

Without this change, milestones will be considered a number, and the trailing 0 (for example in 13.10) will be dropped (and hence result in the milestone being considered 13.1, which is wrong).

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