PHP 8.4 - Fix sundry warnings, update some dependencies

[totten]

This is prep for PHP 8.4. It includes a few direct patches for new warnings, and it also updates a bunch of dependencies. A few notable effects:

• Raise the PHP minimum from 7.1 to 7.3.
• Drop some patches/overrides for marcj/topsort (which are no longer needed)

[totten]

• I took a stab at updating the CI rules to also run tests on php84. However, I had trouble -- I suspect it's because php84 is still in alpha, and the relevant Github Actions haven't been updated in a bit. Maybe we try again in the future.
• However, I have done some local QA on php84:
  • Run phpunit locally ==> OK
  • Run some loco actions (start/stop/clean/etc) in the context of civicrm-buildkit ==> OK