ci(all): fixup failing CI pipeline

[damienwebdev]

PR Checklist

Please check if your PR fulfills the following requirements:

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

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] 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?

CI is broken for various reasons.

Fixes: N/A

What is the new behavior?

We've

1. Pinned PHP/Composer/Magento versions more correctly for future-compatability.
2. Now run integrations tests against tagged releases as opposed to directly against branches in Github.
3. Cached composer across the pipeline, improving test runs by ~2minutes (down from 5).
4. Matrixed across composer versions as well, because Magento versions didn't use to support composer 2.
5. Moved down from ubuntu-latest to ubuntu-18.04 because of Magento 2.3's PHP constraint. We'll revert once it's EoL.

Does this PR introduce a breaking change?

[x] Yes
[ ] No

BREAKING CHANGE: we no longer test against php7.2. Please use it at your own risk.

Other information