Update dependencies of the pubsub gem

The upstream fork has had some updates but our fork has diverged quite a
lot. Update the Google Client gem and Prometheus gem.

We are seeing messages in the build of our FluentD setup like this:

```
Post-install message from google-gax:
*******************************************************************************
The google-gax gem is officially end-of-life and will not be updated further.

If your app uses the google-gax gem, it likely is using obsolete versions of
some Google Cloud client library (google-cloud-*) gem that depends on it. We
recommend updating any such libraries that depend on google-gax. Modern Google
Cloud client libraries will depend on the gapic-common gem instead.
*******************************************************************************
```

.ruby-version

@@ -1 +1 @@
-2.6.5
+3.3.5

fluent-plugin-gcloud-pubsub-custom.gemspec

@@ -17,10 +17,10 @@ Gem::Specification.new do |gem|
   gem.require_paths = ["lib"]
 
   gem.add_runtime_dependency "fluentd", [">= 0.14.15", "< 2"]
-  gem.add_runtime_dependency "google-cloud-pubsub", "~> 0.30.0"
+  gem.add_runtime_dependency "google-cloud-pubsub", "~> 2.19.0"
 
   # Use the same version constraint as fluent-plugin-prometheus currently specifies
-  gem.add_runtime_dependency "prometheus-client", ">= 2.1.0"
+  gem.add_dependency "prometheus-client", ">= 2.1.0"
 
   gem.add_development_dependency "bundler"
   gem.add_development_dependency "pry"