Revert "Update .NET 9 SDK and resolve warnings (#2502)"

[apolcyn]

This reverts commit cd875d9.

Interop client tests in the grpc/grpc repo have been failing continuously since what looks like this PR.

The first failure was on August 13th at 9:40PM PST. The most recent green run before that was on August 13th at 7:40PM PST.

Reverting to see if this fixes things.

---

Failure is happening in (at least) cloud_to_prod_auth:aspnetcore:default:compute_engine_creds:tls. Exception looks like this:

Unhandled exception: System.InvalidOperationException: Expected false but got true.
   at Grpc.Shared.TestAssets.Assert.IsFalse(Boolean condition) in /var/local/git/grpc-dotnet/testassets/Shared/Assert.cs:line 37
   at Grpc.Shared.TestAssets.InteropClient.CreateCredentialsAsync(Nullable`1 useTestCaOverride) in /var/local/git/grpc-dotnet/testassets/Shared/InteropClient.cs:line 248
   at Grpc.Shared.TestAssets.InteropClient.HttpClientCreateChannel() in /var/local/git/grpc-dotnet/testassets/Shared/InteropClient.cs:line 101
   at Grpc.Shared.TestAssets.InteropClient.Run() in /var/local/git/grpc-dotnet/testassets/Shared/InteropClient.cs:line 76
   at InteropTestsClient.Program.<>c__DisplayClass0_0.<<Main>b__0>d.MoveNext() in /var/local/git/grpc-dotnet/testassets/InteropTestsClient/Program.cs:line 92

[JamesNK]

I don't want to revert as that breaks ASP.NET Core benchmarks CI.

Also, many of the updates here are to resolve warnings from vulnerabilities in packages. For example, Google.Apis.Auth 1.46.0 has a dependency on a vulnerable package. Updating to 1.68.0 fixes it.

[JamesNK]

Replaced by #2512