Rename setfrozentag action to set_milestone_frozen_marker

[AliSoftware]

What does it do?

Rename setfrozentag action to set_milestone_frozen_marker, And also make sure it can handle milestone with custom titles (ones that have more than just the version name in their title but also custom text).

Also adds unit tests on the action (there weren't any).

Why?

Because the name of that action was unclear and not following our conventions:

• The name of the action, as it would be used in our Fastfile, was compacted in a single word without spaces (aka _ in camel_case), i.e. setfrozentag, vs set_frozen_tag, making it not super readable
• The naming and wording was not ideal either, as one could easily think that this action was related to GitHub tags, and nothing indicated that this was related to GitHub milestones instead, and had nothing to do with git tags.

Since we're approaching a new major release (as we have Breaking Changes in our changelog waiting to be released), I thought this would be the perfect occasion to clean this one up while at it.

Checklist before requesting a review

• Run bundle exec rubocop to test for code style violations and recommendations
• Add Unit Tests (aka specs/*_spec.rb) if applicable
• Run bundle exec rspec to run the whole test suite and ensure all your tests pass
• Make sure you added an entry in the CHANGELOG.md file to describe your changes under the appropriate existing ### subsection of the existing ## Trunk section.
• If applicable, add an entry in the MIGRATION.md file to describe how the changes will affect the migration from the previous major version and what the clients will need to change and consider.

[mokagio]

All good to merge functionality wise, but there's a typo in the test descriptions that ought to be addressed for the sake of logs clarity.