Update RELEASE_NOTES.md for 1.5.68 release

[Aaronontheweb]

Summary

• Updates RELEASE_NOTES.md with the 1.5.68 release section (3 bug fixes since 1.5.67)
• Updates Directory.Build.props VersionPrefix to 1.5.68 and PackageReleaseNotes via build.ps1

Changes included in 1.5.68

Akka.IO

• Fix: report Tcp.CommandFailed when a scheduled connect retry throws (#8214) — fixes permanent stall of user actor on Linux when a PlatformNotSupportedException is swallowed by the scheduler during a TCP connect retry (#8195)

Akka.Streams

• Fix: observe discarded stream task faults (#8212) — IgnoreSink, QueueSource, and LazySink now observe internal materialized task faults so they don't surface as UnobservedTaskException (#8209, #8210)

Akka.TestKit

• Fix: wrap outer SynchronizationContext in ActorCellKeepingSynchronizationContext (#8182) — prevents test hangs in downstream consumers such as Akka.Hosting.TestKit under xUnit v3 parallel execution

Test plan

• Verify CI passes
• Confirm Directory.Build.props VersionPrefix is 1.5.68