Ensure we can automate the list of API changes for major release changelog

[GuySartorelli]

For CMS 5.0.0 we used https://github.com/emteknetnz/silverstripe-deprecator/blob/main/src/DeprecationDiffTask.php to automate the list of API breaking changes from 4.x to 5.0.0.

There's some hardcoded references in there and no documentation, so reusing it requires some amount of re-learning at least, if not reworking bits of it to work again.

Ideally this process would be built into cow, so we can just run through the release process docs and run the commands it says to run, with expected results.

Acceptance criteria

• At a minimum, make sure that task can work for the 6.0.0-alpha1 changelog for Release CMS 6 alpha #340
• Ideally implement a command in cow (or include this in the existing changelog command) so that future major releases are more streamlined.
  • Should include turning off linting rules proper-names enhanced-proper-names title-case-style so the linter doesn't get mad
  • Any markdown reserved characters in deprecation notice messages are escaped appropriately

[GuySartorelli]

I got this as done as it was needed for the immediate releases. I can keep using what I have locally for the rest of 6 if needs be, and then come back to this afterward for future majors.