Prepare v1.5.55 release

[Aaronontheweb]

Summary

This PR prepares the v1.5.55 release with important stability and security improvements for Akka.Remote.

Changes Included

Akka.Remote Stability Improvements:

• Harden EndpointWriter against serialization failures ([Backport] Akka.Remote: harden EndpointWriter against serialization failures #7925, fixes Akka.Remote: strengthen serialization exception handling #7922)

Akka.Remote Security Improvements:

• Custom certificate validation with single execution path - fixes mTLS asymmetry bug (Backport: feat(remote): custom certificate validation with single execution path - fixes mTLS asymmetry bug (#7915) #7921, fixes feat(remote): add custom certificate validation callbacks and fix asymmetric hostname validation #7914)
• Fix DotNettySslSetup being ignored when HOCON has valid SSL config (Fix DotNettySslSetup being ignored when HOCON has valid SSL config #7919, fixes SslSettings derived from DotNettySslSetup might never actually gets applied #7917)

Release Preparation

• Updated RELEASE_NOTES.md with v1.5.55 entry
• Updated Directory.Build.props to version 1.5.55

Release Process

After this PR is merged to dev:

1. Sync v1.5 branch with merged changes
2. Create annotated git tag 1.5.55 from v1.5 branch
3. Push tag to upstream - GitHub Actions will handle release creation