update dotnet version in tests to latest (#16198)

Update to the latest dotnet version. Pulumi.Awsx started depending on
that, so using an earlier version makes the tests fail.

This happened a few times now, so we should really fix this in a way
that doesn't require us to hardcode this version. But I want to get the
merge queue unblocked, and this is probably the quickest way to do that.

pkg/codegen/testing/test/helpers.go

@@ -378,4 +378,4 @@ const (
 )
 
 // PulumiDotnetSDKVersion is the version of the Pulumi .NET SDK to use in program-gen tests
-const PulumiDotnetSDKVersion = "3.62.0"
+const PulumiDotnetSDKVersion = "3.63.1"