feat: add supported version matrix

[damienwebdev]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/graycoreio/magento2-github-actions/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

There's no well-defined way (that I've found on the internet) to determine the matrix of underlying technology support for Magento. Most often, even the Magento dev docs are wildly inaccurate.

Fixes: N/A

What is the new behavior?

This adds a stable file (and re-usable action) that can be used in Github Actions to verify behavior of a codebase.

Does this PR introduce a breaking change?

[ ] Yes
[x] No