feat: Deprecate google-api-client in favor of individual service-specific gems

google-api-client/google-api-client.gemspec

@@ -13,9 +13,19 @@ Gem::Specification.new do |gem|
   gem.metadata      = {
     "documentation_uri" => "https://googleapis.dev/ruby/google-api-client/v#{Google::Apis::VERSION}"
   }
+
+  gem.post_install_message = <<~MESSAGE
+    *******************************************************************************
+    The google-api-client gem is deprecated and will likely not be updated further.
+
+    Instead, please install the gem corresponding to the specific service to use.
+    For example, to use the Google Drive V3 client, install google-apis-drive_v3.
+    For more information, see the FAQ in the OVERVIEW.md file or the YARD docs.
+    *******************************************************************************
+  MESSAGE
+
   gem.files         = Dir.glob("lib/**/*") +
                       Dir.glob("generated/**/*.rb") +
-                      Dir.glob("docs/**/*") +
                       Dir.glob("*.md") +
                       [".yardopts", "LICENSE"]