Github Actions spawns too many workflows during development

[laceysanderson]

See Issue #278

This PR:

• Adds a README for github action naming conventions.
• renames existing workflows to match conventions.
• Ensures the existing phpunit (one workflow per environment combo) are only run on 9.x-4.x
• Creates a new matrixed phpunit workflow (tests all environments in a single workflow) for use on all development branches and in PRs

This keeps the current functionality but causes less overhead to github actions during development. This way it's easier for us to find out development jobs, see compatibility at a glance and our jobs are not throttled if we push too often :-)

[codeclimate[bot]]

Code Climate has analyzed commit d0b57d16 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 22.2% (0.0% change).

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