For milestone filters, treat `Any` as `No filter`

What does this MR do?

CE port here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30613

Modify the behaviour of milestone selection via Edit Board modal. Persist null for no filter (any), and -1 for no milestone.

Use Any to specifically mean No filter. Any not-null value (worded better) can be added when we iterate on this.

Note: weight MR is here -> https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14180

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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 #4221 (closed)