Trim the 6.0 workflows

[johnbillion]

Previously: #10165

• The reusable workflow files have already been removed from this branch.
• The e2e test workflow in this branch is already pinned to PHP 8.0.

Trac ticket: https://core.trac.wordpress.org/ticket/64083

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props johnbillion, desrosj.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[github-actions]

Test using WordPress Playground

The changes in this pull request can previewed and tested using a WordPress Playground instance.

WordPress Playground is an experimental project that creates a full WordPress instance entirely within the browser.

Some things to be aware of

• All changes will be lost when closing a tab with a Playground instance.
• All changes will be lost when refreshing the page.
• A fresh instance is created each time the link below is clicked.
• Every time this pull request is updated, a new ZIP file containing all changes is created. If changes are not reflected in the Playground instance,
  it's possible that the most recent build failed, or has not completed. Check the list of workflow runs to be sure.

For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

Test this pull request with WordPress Playground.

[desrosj]

@johnbillion What do you think of also trimming 8.1 here? It's set to allow-failures. The release did have beta support. But if the suite does not fully pass to begin with, is it worth keeping those combinations?

I reviewed this one after th 5.9 PR (#11136). But the same question goes for that branch also.

[desrosj]

How did this one get left here. This is a real old iteration. 😅