This backports several build and test tool improvements to the 4.5 branch. Most notably, this includes:
- The changes required to allow each workflow to be triggered by the `workflow_dispatch` event so that tests can be run on a schedule [50590].
- Splitting single site and multisite tests into parallel jobs [50379].
- Split slow tests into separate, parallel jobs for PHP <= 5.6 [50444].
- Better branch and path scoping for GitHub Action workflows when running on `pull_request` [50432,50479].
- Several `devDependency` updates.
Merges [50379,50387,50416,50432,50435,50436,50444,50446,50473,50474,50476,50479,50485,50486,50487,50545,50579,50590] to the 4.5 branch.
See #50401, #51801, #51802, #52548, #52612, #52624, #52625, #52645, #52653, #52658, #52660, #52667.
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