Update RELEASE_NOTES.md for 1.5.9 release

[Arkatufus]

1.5.9 July 18th 2023

Akka.NET v1.5.9 is a maintenance release that introduces some performance improvements and internal code cleanup/modernization.

Changes

• Remoting: Make transport adapter component public

Improvements

• Memory optimization, use Array.Empty instead of creating empty arrays
• Remoting: Log all wrapped message layers during errors
• Port #6805 and #6807, Improve Stream and Pattern.Backoff instance creation time performance
• DData: Harden LWWDictionary serialization null check

Code modernization

• Use C# 9.0 target-typed new()
• Use C# 8.0 null-coalescing operator

Update dependency versions

• Bump Google.Protobuf to 3.23.4
• Bump Akka.MultiNode.TestAdapter to 1.5.8
• Bump Microsoft.Data.SQLite to 7.0.9
• Bump Microsoft.Extensions.ObjectPool to 7.0.8
• Bump Xunit to 2.5.0

Akka.TestKit.Xunit Changes

Due to breaking API change in Xunit 2.5.0, updating to Akka.NET 1.5.9 might break your unit tests. Some of the breaking change that we've noticed are:

• AkkaEqualException constructor has been changed due to changes in Xunit API. If you're using this class, please use the AkkaEqualException.ForMismatchedValues() static method instead of using the constructor.
• Testing for exception types by calling async code inside a sync delegate will not unwrap the AggregateException thrown. Either use async all the way or manually unwrap the exception.
• Xunit Asset.Equal() does not automatically check for collection item equality anymore, that means doing Assert.Equal() between two dictionary or list would not work anymore.
• Some Xunit classes have been changed from public to private. If you're using these classes, you will need to refactor your code.
• FsCheck.Xunit: Xunit Roslyn analyzer has become a bit too overzealous and insists that all unit test method can only return either void or Task and will raise a compilation error if you tried to return anything else. If you're using FsCheck.Xunit, you will need to use a pragma to disable this check: #pragma warning disable xUnit1028.

If you want to see the full set of changes made in Akka.NET v1.5.9, click here.

COMMITS	LOC+	LOC-	AUTHOR
12	171	155	dependabot[bot]
7	466	165	Aaron Stannard
4	1648	1725	Simon Cropp
1	9	4	Gregorius Soedharmo
1	7	1	Michael Buck

[Aaronontheweb]

LGTM