Build/Test Tools: Trim the 6.7 workflows.

This removes reusable workflows that are no longer referenced in the branch, and trims the unit testing matrix to the highest and lowest supported version of each major PHP branchand database version.

Props desrosj, johnbillion

See #64083


git-svn-id: https://develop.svn.wordpress.org/branches/6.7@61736 602fd350-edb4-49c9-b593-d223f7449a82

Author: johnbillion