Restore getLevelName but deprecate it

src/Monolog/Logger.php

@@ -365,6 +365,23 @@ class Logger implements LoggerInterface, ResettableInterface
         }
     }
 
+    /**
+     * Gets the name of the logging level as a string.
+     *
+     * This still returns a string instead of a LevelName for BC, but new code should not rely on this method.
+     *
+     * @throws \Psr\Log\InvalidArgumentException If level is not defined
+     *
+     * @phpstan-param  Level     $level
+     * @phpstan-return LevelName
+     *
+     * @deprecated Use Monolog\Level->toLevelName()->value instead
+     */
+    public static function getLevelName(int|Level $level): string
+    {
+        return self::toMonologLevel($level)->toLevelName()->value;
+    }
+
     /**
      * Converts PSR-3 levels to Monolog ones if necessary
      *