PHP 8.3 compatibility

[falkenhawk]

ensure PHP 8.3 compatibility.

1. fix dynamic property deprecation warning on Framework_AssertTest::$html (cherry-picked from PHP 8.2: Dynamic Properties are deprecated #18)

2. add PHP 8.3 to CI matrix, update actions/checkout v2 → v3

3. move PEAR from composer dependency to pear CLI, switch to composer v2.
   the composer v2 packagist version of pear/pear requires PHP 5.4+, and the PHP 5.3-compatible version previously installable via composer v1 is no longer available since packagist dropped composer v1 support (see discussion in Switch PEAR dependency to github #20).
   In the CI workflow, PEAR is now installed via pear CLI provided by setup-php. Locally, .phpt tests skip gracefully when PEAR is not available.

4. update README with PHP 8.3 compat range

5. fix PHP 8.3 deprecations in GlobalState:

   • ReflectionProperty::setValue() single-arg form deprecated for static properties - use two-arg form with null
   • blacklist _PEAR_Config_instance from globals backup/restore to avoid dynamic property deprecation when unserializing PEAR_Registry objects

6. enable deprecation reporting on CI (error_reporting=E_ALL) - setup-php defaults to production ini which excludes E_DEPRECATED