Update RELEASE_NOTES.md for 1.5.61 release

[Arkatufus]

1.5.61 February 26th, 2026

Akka.NET v1.5.61 is a maintenance release with important bug fixes for Akka.Cluster.Sharding, Akka.Cluster, and Akka.Core.

Akka.Cluster.Sharding Bug Fixes

• Port Pekko ShardStopped handler + handoff safety net - Fixes issue #7500. Resolves a critical issue where shards could fail to hand off indefinitely during scale-up events.
• Fix Shard remember-entities flag mismatch causing entity restart failures - Fixes a bug where the Entities class was initialized with the wrong RememberEntities flag.
• Correct self-comparison in ShardCoordinator ResendShardHost handler - Fixes a bug where the ResendShardHost handler compared a region variable to itself instead of comparing against the message's region.

Akka.Cluster Bug Fixes

• VectorClock inequality fixes - Fixes the != operator which incorrectly delegated to IsConcurrentWith instead of being the logical negation of ==, and fixes the < operator to correctly exclude the equal case.
• Correct format index in 3-arg LogInfo overload - Fixes a logging bug where the 3-arg LogInfo overload used the wrong format index.

Bug Fixes

• Fix wrong randomFactor argument type on RetrySupport.Retry() - Fixes issue #8059. Changes the randomFactor parameter type from int to double to match the expected behavior for jitter calculations.
• AppVersion.CompareTo missing else if breaks comparison symmetry - Fixes a bug where release versions appeared less than their pre-release counterparts.
• Remove stray dollar signs from interpolated strings - Fixes three string interpolation bugs in output in ClusterHeartbeat, ShardRegion, and SinkRefImpl.

Improvements

• Downgrade VirtualPathContainer RemoveChild log from Warning to Debug - Fixes issue #8037. Eliminates noisy warnings during high-load Ask operations.

4 contributors since release 1.5.60

COMMITS	LOC+	LOC-	AUTHOR
4	2085	27	Aaron Stannard
5	52	14	Matt Kotsenas
3	151	9	Gregorius Soedharmo
1	2	2	Apoorv Darshan

To see the full set of changes in Akka.NET v1.5.61, click here.