Trim the GitHub Actions workflows for older branches

[johnbillion]

For older branches (ie. prior to 6.8 currently) I propose:

1. Unused workflow files (mostly the reusable ones) are trimmed. This has been done previously on an ad-hoc basis, this is just catching up recent branches.
2. The performance.yml workflow is additionally removed. Realistically this is not something that anybody looks at outside of trunk and the current major, and results for branches aren't available on codevitals.run.
3. The matrices for PHPUnit tests are reduced to boundary testing of each major PHP version, which means the highest and lowest supported version in each of 5, 7, and 8. This reduces the time for tests to run and reduces the number of minutes consumed by workflows. This is especially useful when preparing backports to multiple branches.

For the 6.7 branch:

• Deletes unnecessary workflows
• Trims the unit testing matrix to the highest and lowest supported version of each major PHP branch.