Fix deployment.concurrency_limit always null.

[collincchoy]

PrefectHQ/prefect#15426 includes some API changes that results in the deployment details always showing the concurrency limit as None.

That PR:

• adds a full GCL/concurrencylimitv2 onto deployment responses that have a concurrency limit
• makes the top-level concurrency_limit value always null -- would remove but keeping it for backwards compatibility.

The changes here should be backwards compatible in that it uses concurrency_limit if present else falls back to the nested limit value else null.