Update `com_google_protobuf` version to `v3.21.9`.

* Updated `com_google_protobuf` version, which no longer depends on `six`.
* Removed dependency on `six`.
* Updated some comments.

PiperOrigin-RevId: 491688041

WORKSPACE

@@ -33,7 +33,7 @@ git_repository(
     # Patched to give visibility into private targets to pybind11_protobuf
     patches = ["//third_party/protobuf:com_google_protobuf_build.patch"],
     remote = "https://github.com/protocolbuffers/protobuf.git",
-    tag = "v3.18.0-rc1",
+    tag = "v3.21.9",
 )
 
 git_repository(
@@ -84,7 +84,7 @@ git_repository(
 # Inlined transitive dependencies, grouped by direct dependency.
 #
 
-# Required by pybind11_bazel
+# Required by pybind11_abseil and pybind11_protobuf
 new_git_repository(
     name = "pybind11",
     build_file = "@pybind11_bazel//:pybind11.BUILD",
@@ -92,22 +92,6 @@ new_git_repository(
     tag = "v2.9.2",
 )
 
-# Required by pybind11_bazel
-# load("@pybind11_bazel//:python_configure.bzl", "python_configure")
-# python_configure(name = "local_config_python")
-
-# Required by absl_py
-http_archive(
-    name = "six",
-    build_file = "//third_party:six.BUILD",
-    sha256 = "1e61c37477a1626458e36f7b1d82aa5c9b094fa4802892072e49de9c60c4c926",
-    strip_prefix = "six-1.16.0",
-    urls = [
-        "https://storage.googleapis.com/mirror.tensorflow.org/pypi.python.org/packages/source/s/six/six-1.16.0.tar.gz",
-        "https://pypi.python.org/packages/source/s/six/six-1.16.0.tar.gz",
-    ],
-)
-
 #
 # Transitive dependencies, grouped by direct dependency.
 #