Release Google.Api.Gax version 3.4.0

Changes since 3.3.0:

- Allow an IHttpClientFactory to be specified in REST ClientBuilders
- gRPC libraries updated to 2.38.0
- Google.Apis.Auth updated to 1.52.0
- Support GKE on Windows as far as we can in `Platform`
- Improved derivation of the GKE container name from mountinfo
- Avoid a redirect when checking the metadata server

ReleaseVersion.xml

@@ -5,6 +5,6 @@
     - divergent versions, but for the moment this keeps things simpler.
     -->
   <PropertyGroup>
-    <Version>3.4.0-alpha01</Version>
+    <Version>3.4.0</Version>
   </PropertyGroup>
 </Project>