Update RELEASE_NOTES.md for 1.5.63 release

[Arkatufus]

1.5.63 March 23rd, 2026

Akka.NET v1.5.63 is a maintenance release that includes a critical Akka.Remote bug fix along with Akka.Streams fixes and a major migration of all test projects to xUnit 3.
All users running Akka.Remote or Akka.Cluster are strongly encouraged to upgrade.

• Fix stale ACK causing irrecoverable quarantine after transient network disruption - Fixes stale ACK could cause an irrecoverable quarantine.
• Fix race condition in QueueSource offer handling - Fixes a race condition in QueueSource.
• Fix race condition in UnfoldResourceAsyncSource callback invocation - Fixes a race condition in UnfoldResourceAsyncSource.
• Remove FluentAssertions from Akka.Persistence.TCK and Akka.Persistence.TCK.Xunit2 - Removes FluentAssertions dependency from the persistence TCK packages.
• Migrate all test projects to xUnit 3 - Complete migration of all Akka.NET test projects to xUnit 3.

Important Akka.Remote Bug Fix

Fixes a critical issue where a stale ACK from a previous connection could cause an irrecoverable quarantine state after a transient network disruption, permanently preventing nodes from re-establishing communication.
In affected scenarios, the only recovery option was a full restart of the quarantined node. This fix ensures that stale ACKs are correctly discarded during reconnection, allowing nodes to recover automatically after network interruptions.

2 contributors since release 1.5.62

COMMITS	LOC+	LOC-	AUTHOR
28	4009	1637	Gregorius Soedharmo
2	42	21	Aaron Stannard

To see the full set of changes in Akka.NET v1.5.63, click here.