Enable Memory Watchdog by default for puma

In #379198 (closed) we introduced RssMemoryLimit monitor for watchdog wich is backward compatible with puma. In case our customers are using PWK, we will move them to Watchdog, but they will use RssMemoryLimit, which uses puma[per_worker_max_memory_mb] if configured.

In order to move customers to watchdog we need to enable GITLAB_MEMORY_WATCHDOG_ENABLED by default for Puma. We also want to add a check if watchdog is enabled in both PumaWorkerKillerInitializer and memory_watchdog initializer.

In case watchdog is enabled, we don't initialize PWK. In case customers are not using PWK, we use watcdog with default puma motnitors: HeapFragmentation and UniqueMemoryGrowth, same as for SaaS.