allow overriding the minimum PHP release for rector

By setting the environment variable RECTOR_MIN_PHP the supported minimum
PHP release can be set. This is needed to make plugin changes compatible
with DokuWiki stable.

Author: splitbrain

_test/rector.php

@@ -62,9 +62,14 @@
     $rectorConfig->cacheClass(FileCacheStorage::class);
     $rectorConfig->cacheDirectory(__DIR__ . '/.rector-cache');
 
+    // supported minimum PHP version can be overridden by environment variable
+    [$major, $minor] = explode('.', $_SERVER['RECTOR_MIN_PHP'] ?? '7.4');
+    $phpset = LevelSetList::class . '::UP_TO_PHP_' . $major . $minor;
+    echo "Using PHP set $phpset\n";
+
     // define sets of rules
     $rectorConfig->sets([
-        LevelSetList::UP_TO_PHP_74,
+        constant($phpset),
         SetList::CODE_QUALITY,
         SetList::DEAD_CODE,
         SetList::CODING_STYLE,