[meta] Remove dead code/unneeded compatibility layers now that PHP 8.1 is required

Parenting to Drupal 10 beta1 issue.

Union types are (will be?) a thing in PHP 8:

https://wiki.php.net/rfc/union_types_v2

Maybe this is a good opportunity to go through and add complete return types to most/all functions/methods?

Thanks Andy :)

I *think* this issue was meant to be a cleanup of backwards compatibility code that is left for PHP 7 support / forward compatibility with PHP 8. That is how it got into the Drupal 10 beta requirements.

#3252088: Increase Drupal::MINIMUM_PHP to 8.0.0 landed today.

(That does not in itself means that issues here should be pushed forward, depending on how much divergence should be in Drupal 9.4 vs. Drupal 10. Best to get release manager confirmation here.)

Now that alpha1 is tagged and we're starting general deprecation removals, we can probably go ahead with this work as well.

Oh, re: #6, we would start adding union types and similar to 10.1.x at the earliest. Drupal 9.4 and Drupal 10 need to have the same public API, which means that public API needs to be compatible with PHP 7.3. Futhermore, adding typehints to existing code would need to be done with BC and deprecation.

Just reiterating that work in this issue should be limited to removing dead code paths where older PHP versions are handled separately. PHPUnit often requires us to add such compatibility layers, for example.

Added two child issues:

#2532228: Use RecursiveCallbackFilterIterator instead of the $acceptTests parameter forwarding - PHP 5.3!
#3001920: Investigate PHP 7.3 workarounds

The MySQL driver has some comments about PHP 7:

    if (!$this->connection->inTransaction()) {
      // On PHP 7 $this->connection->inTransaction() will return TRUE and
      // $this->connection->rollback() does not throw an exception; the
      // following code is unreachable.

    if ($this->connection->inTransaction()) {
      // On PHP 7 $this->connection->inTransaction() will return TRUE and
      // $this->connection->commit() does not throw an exception.

Not sure we have an issue for these.

I believe #3310307: GDToolkit::save() fails for some custom-local stream wrappers fits the bill of "remove unneeded compatibility layers".

Though I'm not claiming it's a must-have for D10, because AFAICT it could have been removed for PHP7 / D9 already and apparently noone encounters this issue...

I came across #1210798: In PHP 5.4+, html_entity_decode() doesn't decode invalid numeric entities which lead me to discover there are checks for PHP v 5.4 in core/modules/editor/tests/src/Unit/EditorXssFilter/StandardTest.php and core/tests/Drupal/Tests/Component/Utility/XssTest.php.

I haven't made an issue to remove those checks (I am focusing on something else right now) but noting it here in the hopes I come back or someone else picks this up.

Created #3321945: Remove PHP < 5.4.0 checks to deal with the PHP 5.4 checks as discovered by @quietone in the comment above me.