refactor: PHP 8.5 language features via Rector

rector.php gets the PHP sets up to 8.5. Mostly constructor promotion with
readonly properties, typed class constants, ??=, arrow functions,
str_contains/str_starts_with, catch without unused variables, array_any
and #[\Override] on overridden properties; array_key_exists() keys are
cast to string (null keys are deprecated in PHP 8.5). Long promoted
constructor signatures are wrapped one parameter per line.

Checked locally: a crawl of ~1,900 frontend URLs over all sites and the
backchannel APIs (/api/hotel, /api/product, content feed manifest and
records of every type) render the same as before.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

rector.php

@@ -27,6 +27,7 @@ return RectorConfig::configure()
         MigratePluginContentElementAndPluginSubtypesTCARector::class,
     ])
     ->withPhpVersion(PhpVersion::PHP_85)
+    ->withPhpSets(php85: true)
     ->withSets([
         Typo3SetList::GENERAL,
         Typo3LevelSetList::UP_TO_TYPO3_13,