Bring GitHub Actions workflows back to working state

[mdickinson]

PR updating and fixing our CI for newest Ubuntu and other changes:

• [x] Fix apt-get package list for Ubuntu-22.04
• [x] Move OS package installation into a separate composite action
• [x] Replace uses of GabrielBB/xvfb-action action with xvfb-run
• [x] Drop matrix entries that require Python 3.6 from actions/setup-python
• [x] Drop PySide2 testing in non-EDM tests (test only on PySide6)
• [x] Bypass copyright year check for now, to avoid cluttering this PR with copyright fixes; I'll open another issue for this. (#492)

We have three tests that have been causing problematic segfaults: #476. Those are increasingly interfering with CI runs, even for changes that have nothing to do with the relevant code, so I'm going to skip those tests unconditionally in CI + Linux. (That's far from ideal, but I don't see a better solution.)

[mdickinson]

Test run for ets-with-source cron job: https://github.com/enthought/envisage/actions/runs/3846537542