Increase Drupal 10's RECOMMENDED_PHP to 8.1 now

Problem/Motivation

Related issues:

• #3223443: [policy, no patch] Process for dealing with EOL PHP versions during the Drupal 10 and future release cycles
• #3223435: Track PHP 8.1 support in hosting and distributions
• #3173787: [policy] Require PHP 8.1 for Drupal 10.0 if a dependency does, otherwise require PHP 8.0. (up from 7.3 in Drupal 9)
• #3252386: Use PHP attributes instead of doctrine annotations

In all of the above, we are still exploring whether we should (or even can) require PHP 8.1 for Drupal 10. However, we certainly can and should recommend it. There are a few PHP 8.1 issues still outstanding, but they are mostly feature requests or notices/deprecation warnings. The hard blockers for PHP 8.1 use (e.g. upstream dependency incompatibilities and fatals) have (mostly? entirely?) already been resolved in Drupal 9.3.

Proposed resolution

• Increase RECOMMENDED_PHP to 8.1 now, to set expectations that it's still the best choice for D10 even though we might not be able to require it yet.
• General changes to how the recommended PHP version works should be part of followups in #3223443: [policy, no patch] Process for dealing with EOL PHP versions during the Drupal 10 and future release cycles.

Remaining tasks

• Update the system requirements docs accordingly.
• Decide whether to additionally backport this recommendation to 9.4.

User interface changes

Users on PHP 8.0 will see the normal messaging about PHP 8.1 being the recommended version.

API changes

None.

Data model changes

None.

Release notes snippet

PHP 8.1 is now the recommended PHP version to use with Drupal 10. (It may be the required PHP version before 10.0.0 or in a future release.)