Add 8.4 stable and 8.5-dev to test matrix

[Firehed]

PHP 8.4 was released last week, so this updates the test matrix accordingly. 8.1 can be reasonably dropped as well.

8.4 and 8.5 are now emitting a deprecation warning from the ASN.1 library. I intend to send a PR, but there's been no activity for quite some time (understandable given the context of the library, but there's also an open PR for other language-related improvements) so switching to a more actively-maintained library may end up being a better choice.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.26%. Comparing base (1031cc6) to head (02d65ae).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main      #88   +/-   ##
=========================================
  Coverage     81.26%   81.26%           
  Complexity      308      308           
=========================================
  Files            33       33           
  Lines          1025     1025           
=========================================
  Hits            833      833           
  Misses          192      192           

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