Fix ambiguous query error on milestone_id

What does this MR do?

This MR updates the scope in the Milestoneable concern to reference the column according to our guidelines: https://docs.gitlab.com/ee/development/sql.html#reliably-referencing-database-columns

We noticed a problem where one of the Value Stream Analytics query raised the following error:

ActiveRecord::StatementInvalid - PG::AmbiguousColumn: ERROR: column reference "milestone_id" is ambiguous

The query joins issues, merge_requests_closing_issues and merge_requests tables and adds a filter on milestone_id. The column is available in both issues and merge_requests tables.

Related discussion: !37405 (comment 382700018)

I added an special test case that produces a similar query and asserts that the query is not raising error.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
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• [-] 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
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