Add release notes for 2.7.4

Author: NickCraver

docs/ReleaseNotes.md

@@ -8,6 +8,10 @@ Current package versions:
 
 ## Unreleased
 
+No pending unreleased changes.
+
+## 2.7.4
+
 - Adds: RESP3 support ([#2396 by mgravell](https://github.com/StackExchange/StackExchange.Redis/pull/2396)) - see https://stackexchange.github.io/StackExchange.Redis/Resp3
 - Fix [#2507](https://github.com/StackExchange/StackExchange.Redis/issues/2507): Pub/sub with multi-item payloads should be usable ([#2508 by mgravell](https://github.com/StackExchange/StackExchange.Redis/pull/2508))
 - Add: connection-id tracking (internal only, no public API) ([#2508 by mgravell](https://github.com/StackExchange/StackExchange.Redis/pull/2508))