Run tests on PHP 8.4 and update test environment

[PaulRotmann]

Builds on top of #267, #261, #255 and others.

⚠️ This PR can only be merged after PR #267 has been merged, as it requires the PHP 8.4 compatibility fixes and dependency updates to pass successfully.

Also this PR resolves #266.

[clue]

@PaulRotmann Thanks for looking into this, changes look reasonable 👍

However, besides the PHPstan update discussed in #267, the test suite currently reports one error on PHP 8.4+ only:

There was 1 failure:

1) /home/runner/work/promise/promise/tests/FunctionSetRejectionHandlerThatTriggersDefaultHandlerShouldTerminateProgramForUnhandled.phpt
Failed asserting that string matches format description.
--- Expected
+++ Actual
@@ @@
-Fatal error: Unexpected RuntimeException: foo in %s line %d
+Deprecated: Passing E_USER_ERROR to trigger_error() is deprecated since 8.4, throw an exception or call exit with a string message instead in /home/runner/work/promise/promise/tests/FunctionSetRejectionHandlerThatTriggersDefaultHandlerShouldTerminateProgramForUnhandled.php on line 9
+
+Fatal error: Unexpected RuntimeException: foo in /home/runner/work/promise/promise/tests/FunctionSetRejectionHandlerThatTriggersDefaultHandlerShouldTerminateProgramForUnhandled.php on line 9

/home/runner/work/promise/promise/tests/FunctionSetRejectionHandlerThatTriggersDefaultHandlerShouldTerminateProgramForUnhandled.phpt:24

Here's some background info:

Calling trigger_error() with error_level being equal to E_USER_ERROR is now deprecated.

— https://www.php.net/manual/en/migration84.deprecated.php

This only affects a single test that specifically tests how the rejection logic behaves when a custom function using this (deprecated) functionality is used. Accordingly, we should probably skip the affected test on PHP 8.4+ (originally introduced in #249):

promise/tests/FunctionSetRejectionHandlerThatTriggersDefaultHandlerShouldTerminateProgramForUnhandled.phpt

Line 15 in 5f80055

trigger_error('Unexpected ' . get_class($e) . ': ' .$e->getMessage(), E_USER_ERROR);