Feat(datetime utility): add diff months util

This MR adds a difference in months utility to the datetime_utility file. This will, specifically, be used for charts on the Instance Analytics page, but will probably be useful for other places as well.

Related MR: !44927 (merged)

Screenshots

N/A.

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

Not much risk as this is not being used, yet, but tests have also been added to ensure correct behavior.

• 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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #246492 (closed) and #268224 (closed)