.NET9 support

[Kielek]

Why

Towards #3766

What

Adds support for .NET9.
Drops support for .NET6 and .NET7.
Other related cleanup.
Bumping .NET SDK to 8.0.404
LangVersion updated to 13 (version supported by .net 9). previous version was taken from .NET7 (needed for some genrated, grpc code).

Tests

CI

Checklist

• CHANGELOG.md is updated.
• Documentation is updated.
• New features are covered by tests.

Notes

dotnet format is failing. Not related to changes.
PR is huge, IMO worth to review commit by commit.

[Kielek]

@stevejgordon, fyi

[zacharycmontoya]

I've looked through and overall it looks good. I'll wait for resolution on existing comments before approving

[nrcventura]

There are many places in the code where we needed to change NET60_OR_GREATER to NET80_OR_GREATER. It seems like the intent of the directive in many, if not most of those locations was to use a version of the code that only applied to the .net build target and not the .net framework build target.

Where appropriate, should we change NET80_OR_GREATER to either NET or !NETFRAMEWORK, to make the intent clearer?

[Kielek]

There are many places in the code where we needed to change NET60_OR_GREATER to NET80_OR_GREATER. It seems like the intent of the directive in many, if not most of those locations was to use a version of the code that only applied to the .net build target and not the .net framework build target.

Where appropriate, should we change NET80_OR_GREATER to either NET or !NETFRAMEWORK, to make the intent clearer?

@nrcventura, I agree, but I do not want to make such changes in the same file. just kept the current convention. New issue created: #3798 (not a blocker to the release IMO, just normal maintenance task).

[RassK]

Some minor things and tweaks only. In overall tests are passing, seems like good to go.

[zacharycmontoya]

LGTM, thanks!