[end2end] Move test to TLSv1.3 as advertised (#30574)

We spotted this during review of #30569. Not wanting to make a behavior change there I left it as is, but the test advertises TLSv1.3 so it seems likely this should be changed.

test/core/end2end/fixtures/h2_tls_certwatch_async_tls1_3.cc

@@ -38,7 +38,7 @@ chttp2_create_fixture_async_verifier_cert_watcher(const grpc_channel_args*,
   fullstack_secure_fixture_data* ffd = new fullstack_secure_fixture_data();
   memset(&f, 0, sizeof(f));
   ffd->localaddr = grpc_core::JoinHostPort("localhost", port);
-  SetTlsVersion(ffd, SecurityPrimitives::TlsVersion::V_12);
+  SetTlsVersion(ffd, SecurityPrimitives::TlsVersion::V_13);
   SetCertificateProvider(ffd, SecurityPrimitives::ProviderType::FILE_PROVIDER);
   SetCertificateVerifier(
       ffd, SecurityPrimitives::VerifierType::EXTERNAL_ASYNC_VERIFIER);