Add matrix to Test workflow

[AJGranowski]

What are these changes?

This change updates the Test workflow to run on ubuntu-latest, ubuntu-24.04, and ubuntu-22.04.
This change also addresses some issues I encountered while making that change.

Why are these changes being made?

• Update megalinter to v7.12.0. This updates actionlint to v1.7.1, which adds support for the ubuntu-24.04 runner label.
• Update poetry.lock to address some CVEs.
• Update ScribeMD/pre-commit-action to fix errors about running on an old Node version.
• Split the test job into pre-commit, test-action, and notify. This allows us to test the action on a matrix of runners without running ScribeMD/pre-commit-action and ScribeMD/slack-templates multiple times.
• Remove ubuntu-18.04 and ubuntu-20.04 from the README since ubuntu-18.04 is no-longer supported, and this action doesn't work on ubuntu-20.04.

Notes

• This PR depends on #402.