[BUILD] Use bazel-skylib rule to check bazel version (#20990)

Bazel has a rule for enforcing the version so we can just reuse that.

This redundant bazel version check logic in setup.py is also causing issue when building conda package, because conda has its own version of bazel and it doesn't support `--version`.

WORKSPACE

@@ -17,4 +17,6 @@ grpc_extra_deps()
 
 load("@bazel_skylib//lib:versions.bzl", "versions")
 
-versions.check(minimum_bazel_version = "3.4.0")
+# When the bazel version is updated, make sure to update it
+# in setup.py as well.
+versions.check(minimum_bazel_version = "4.2.1")

python/setup.py

@@ -13,7 +13,6 @@ import tempfile
 import zipfile
 
 from itertools import chain
-from itertools import takewhile
 from enum import Enum
 
 import urllib.error
@@ -23,6 +22,8 @@ import urllib.request
 logger = logging.getLogger(__name__)
 
 SUPPORTED_PYTHONS = [(3, 6), (3, 7), (3, 8), (3, 9)]
+# When the bazel version is updated, make sure to update it
+# in WORKSPACE file as well.
 SUPPORTED_BAZEL = (4, 2, 1)
 
 ROOT_DIR = os.path.dirname(__file__)
@@ -485,17 +486,6 @@ def build(build_python, build_java, build_cpp):
             ] + pip_packages,
             env=dict(os.environ, CC="gcc"))
 
-    version_info = bazel_invoke(subprocess.check_output, ["--version"])
-    bazel_version_str = version_info.rstrip().decode("utf-8").split(" ", 1)[1]
-    bazel_version_split = bazel_version_str.split(".")
-    bazel_version_digits = [
-        "".join(takewhile(str.isdigit, s)) for s in bazel_version_split
-    ]
-    bazel_version = tuple(map(int, bazel_version_digits))
-    if bazel_version < SUPPORTED_BAZEL:
-        logger.warning("Expected Bazel version {} but found {}".format(
-            ".".join(map(str, SUPPORTED_BAZEL)), bazel_version_str))
-
     bazel_flags = ["--verbose_failures"]
     if BAZEL_LIMIT_CPUS:
         n = int(BAZEL_LIMIT_CPUS)  # the value must be an int