Upmerge 1.44.x branch into master (#28700)

* xDS: Rbac filter updates (#28568) (#28608)

* Bump version to v1.44.0-pre1 (on the release branch) (#28593)

* bump version to 1.44.0-pre1

* regenerate projects

* only apply "singleplatform" nuget suffix when actually needed (#28677)

* Bump version to 1.44.0-pre2 (#28681)

* bump version to 1.44.0-pre2

* regenerate projects

Co-authored-by: Yash Tibrewal <yashkt@google.com>

tools/run_tests/artifacts/package_targets.py

@@ -91,7 +91,7 @@ class CSharpPackage:
         del inner_jobs  # arg unused as there is little opportunity for parallelizing
         environ = {
             'GRPC_CSHARP_BUILD_SINGLE_PLATFORM_NUGET':
-                os.getenv('GRPC_CSHARP_BUILD_SINGLE_PLATFORM_NUGET')
+                os.getenv('GRPC_CSHARP_BUILD_SINGLE_PLATFORM_NUGET', '')
         }
         if self.unity:
             return create_docker_jobspec(