- All workspace suffix are removed from WORKSPACE.bzlmod, since we get
  all dependencies and toolchains from Bzlmod.
- Added build:bzlmod bazelrc section, we have to override two flags to
  make sure toolchains are picked up correctly.
- Added zstd-jni as dependency
- Added postsubmit_bzlmod.yml for testing the Bzlmod build.

Tested at https://buildkite.com/bazel/bazel-bazel-with-bzlmod/builds/3

Closes #14312.

PiperOrigin-RevId: 411855954