chore(internal): update microgen to v0.21.2 (#4315)

Changes include:
- add initial `rest` client generation (behind `transport=rest` flag)
- add `EnableJwtWithScope` default client options cc @shinfan
- dependency updates

internal/gapicgen/cmd/genbot/Dockerfile

@@ -28,7 +28,7 @@ RUN GO111MODULE=on go get \
     golang.org/x/lint/golint@latest \
     golang.org/x/tools/cmd/goimports@latest \
     honnef.co/go/tools/cmd/staticcheck@latest \
-    github.com/googleapis/gapic-generator-go/cmd/protoc-gen-go_gapic@v0.20.4
+    github.com/googleapis/gapic-generator-go/cmd/protoc-gen-go_gapic@v0.21.2
 ENV PATH="${PATH}:/root/go/bin"
 
 # Source: http://debuggable.com/posts/disable-strict-host-checking-for-git-clone:49896ff3-0ac0-4263-9703-1eae4834cda3