Support building TensorBoard with Bazel version >= 7.0.0

TensorBoard won't build with the most recent Bazel 7.0.0, released yesterday, due to an error arising from rules_apple (note: only visible after bumping the Bazel allowed version window in WORKSPACE):

ERROR: Traceback (most recent call last):
	File "/usr/local/google/home/nickfelt/.cache/bazel/_bazel_nickfelt/3cdf037ab9be95ef9846209ffcda115e/external/build_bazel_rules_apple/apple/internal/rule_support.bzl", line 226, column 36, in <toplevel>
		deps_cfg = apple_common.multi_arch_split,
Error: 'apple_common' value has no field or method 'multi_arch_split'
...
ERROR: error loading package '@@com_github_grpc_grpc//src/compiler': at /usr/local/google/home/nickfelt/.cache/bazel/_bazel_nickfelt/3cdf037ab9be95ef9846209ffcda115e/external/com_github_grpc_grpc/bazel/grpc_build_system.bzl:33:6: at /usr/local/google/home/nickfelt/.cache/bazel/_bazel_nickfelt/3cdf037ab9be95ef9846209ffcda115e/external/build_bazel_rules_apple/apple/ios.bzl:26:5: at /usr/local/google/home/nickfelt/.cache/bazel/_bazel_nickfelt/3cdf037ab9be95ef9846209ffcda115e/external/build_bazel_rules_apple/apple/internal/testing/ios_rules.bzl:30:5: at /usr/local/google/home/nickfelt/.cache/bazel/_bazel_nickfelt/3cdf037ab9be95ef9846209ffcda115e/external/build_bazel_rules_apple/apple/internal/rule_factory.bzl:62:5: initialization of module 'apple/internal/rule_support.bzl' failed
ERROR: /usr/local/google/home/nickfelt/tb/WORKSPACE:197:22: error loading package '@@com_github_grpc_grpc//src/compiler': at /usr/local/google/home/nickfelt/.cache/bazel/_bazel_nickfelt/3cdf037ab9be95ef9846209ffcda115e/external/com_github_grpc_grpc/bazel/grpc_build_system.bzl:33:6: at /usr/local/google/home/nickfelt/.cache/bazel/_bazel_nickfelt/3cdf037ab9be95ef9846209ffcda115e/external/build_bazel_rules_apple/apple/ios.bzl:26:5: at /usr/local/google/home/nickfelt/.cache/bazel/_bazel_nickfelt/3cdf037ab9be95ef9846209ffcda115e/external/build_bazel_rules_apple/apple/internal/testing/ios_rules.bzl:30:5: at /usr/local/google/home/nickfelt/.cache/bazel/_bazel_nickfelt/3cdf037ab9be95ef9846209ffcda115e/external/build_bazel_rules_apple/apple/internal/rule_factory.bzl:62:5: initialization of module 'apple/internal/rule_support.bzl' failed and referenced by '//external:grpc_python_plugin'

Based on https://github.com/bazelbuild/bazel/issues/19938 it looks like this is ultimately an issue with gRPC, which needs to update to support Bazel 7.0.0 including bumping rules_apple (which has released a fix). So far, they don't seem to be actively working on it though: https://github.com/grpc/grpc/issues/34791

Start with the Bazel version window in WORKSPACE and the gRPC dependency referenced at WORKSPACE line 197; reproduce the failure with Bazel 7.0.0 and review the linked gRPC issue and rules_apple fix. Done means TensorBoard builds with Bazel 7.0.0 or newer without the apple_common.multi_arch_split error.