update cohort deprecation text

[JFriel]

Proposed Change

• Improve cohort deprecation replacement text
• When replacing a deprecated cohort usage, ignore frozen ones

Type of change

What types of changes does your code introduce? Tick all that apply.

• Bugfix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that would cause existing functionality to not work as expected)
• Documentation-Only Update
• Other (if none of the other choices apply)

Checklist

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• Ensured that the PR branch is in sync with the target branch (i.e. it is automatically merge-able)
• Created or updated any tests if relevant
• Have validated this change against the Test Plan
• Requested a review by one of the repository maintainers
• Have written new documentation or updated existing documentation to detail any new or updated functionality and how to use it
• Have added an entry into the changelog