Deprecate usage of timebox values in milestone title in API v5

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Context

We have supported using the timebox values as wildcard in the param milestone_title when filtering issues by milestone.

This interface is problematic in that

1. when the title of a milestone exactly matches one of the timebox values, the milestone may get omitted as filtering criterion.

2. it doesn't match the conventions used for other objects like iteration.

milestone_id will be added to support specifying milestone timebox wildcard values in #335939 (closed) and then we can safely remove the current usage of timebox values in milestone_title.