Remove mysql 5.7 from unit tests

[stephanme]

• A short explanation of the proposed change:

Take out mysql 5.7 from the unit test matrix as it is explicitly not supported for cf-deployment.

• An explanation of the use cases your change solves

Allows to use mysql features and rely on bug fixes that are not available in mysql 5.7.
Example: Dropping of duplicated unique constraints in DB migrations.

• Links to any other associated PRs

Fixes #3949 and unblocks #3952

• I have reviewed the contributing guide

• I have viewed, signed, and submitted the Contributor License Agreement

• I have made this pull request to the main branch

• I have run all the unit tests using bundle exec rake

• I have run CF Acceptance Tests

[sethboyles]

Our preference would be to hold off on this until May, as we support foundations using MySQL 5.7 until then.

[stephanme]

OK, we need then to adapt #3949 to work with mysql 5.7.
Labeled it as blocked until May 2025.