Fix failing GitHub e2e tests

[alecgeatches]

As a first step for modernization, this PR:

• Fixes end-to-end tests. Most of these failures were due to old CSS selectors that no longer work with modern WordPress admin components. All existing end to end tests should now pass.
• Updates the lowest supported PHP version to 7.4, and removes testing for lower versions.
• Explicitly uses a .nvmrc to mark the node version of 14. We'll want to move this to a modern version (20) soon.
• Removes eslines from npm lint-js. eslines is an Automattic utility that seemed to be used to ignore linting errors also in the master branch, but was throwing errors and blocking tests. As it's close to 3 years out of date, I just dropped the usage and allowed eslint tests to fail. We'll come back to these very soon to --fix, ignore, and manually fix the remaining errors.

[alecgeatches]

End to end tests are now passing! See the last run: https://github.com/Automattic/Edit-Flow/actions/runs/9165713334/job/25199719325

The lint-js task has been moved to run after jest and end-to-end tests. We'll come back to address the tons of linting warnings and errors very soon.

[alecgeatches]

Hello @rebeccahum! We're doing some refactorings starting with fixing end-to-end tests in GitHub Actions and a PHP version bump. Please take a look! We'll plan to do a new plugin release once we're done with modernization work.

[ingeniumed]

Would it be worth switching the branch it's going to, to be a release branch? That way, once we wrap up our modernization changes we can make a single merge of that into the main branch and cut the release. There's going to be another breaking change after this so figured do it one go.

[alecgeatches]

@ingeniumed I'm going to merge this into a new develop branch. Thank you!