Raise version requirement to PHP 8.

[demiankatz]

This PR raises our minimum version requirement to PHP 8, since PHP 7 has reached end of life.

Next steps (to be done in separate PRs):

• Raise eligible dependency versions (and test)
• Adjust the PHP version in all file headers (probably best done as a separate PR to reduce confusion)
• Add PHP 8.2 to the build matrix and to branch protection rules (it doesn't work as of this writing, but updating the dependencies may help).

[demiankatz]

Note that I've removed the PHP 7.4 build from our branch protection rules, since we won't be able to merge this if a successful 7.4 build is a prerequisite for merges.