[meta] PHP 8.6 support

Problem/Motivation

PHP 8.6 schedule supposed GA release at Nov 19 2026 and the first alpha is out bringing up incompatibilities

- https://wiki.php.net/rfc/deprecate-return-value-from-construct

Steps to reproduce

https://github.com/pear/Archive_Tar/pull/61
https://github.com/phpspec/prophecy/issues/672

    1) /builds/vendor/phpspec/prophecy/src/Prophecy/Doubler/Generator/ClassCreator.php(44) : eval()'d code:15
    Returning a value from a destructor is deprecated
    
    Triggered by:
    
    * Drupal\Tests\Core\Cache\DatabaseBackendFactoryTest::testGet#default
      /builds/core/tests/Drupal/Tests/Core/Cache/DatabaseBackendFactoryTest.php:33

Proposed resolution

As CI already running 8.6 and the polyfill is enable, so create child issues when it fails

Remaining tasks

Fix upcoming issues and wait til release date

Patches

- https://github.com/php/php-src/issues/21691
- https://github.com/phpspec/prophecy/pull/673 and PR#68 from alexpott
- https://github.com/pear/Archive_Tar/pull/61
- https://github.com/krakjoe/pcov/pull/133

User interface changes

no

Introduced terminology

no

API changes

no

Data model changes

no

Release notes snippet

Release is compatible with PHP 8.6