Clean up downstream projects (#2101)

Keep only some important end projects and Bazel team maintained projects
in the downstream pipeline. Removing projects that are deprecated or
non-responsive to breakages and projects that will be covered by the new
[BCR compatibility test
pipeline](https://buildkite.com/bazel/bcr-bazel-compatibility-test).

Non-responsive or deprecated repos:

- https://github.com/bazelbuild/codelabs.git
- https://github.com/bazelbuild/bazel-watcher.git
- https://github.com/googlecartographer/cartographer.git
- https://github.com/envoyproxy/envoy.git
- https://github.com/google/flatbuffers.git
- https://gerrit.googlesource.com/gerrit.git
- https://github.com/kythe/kythe.git
- https://github.com/bazelbuild/rules_closure.git
- https://github.com/bazelbuild/rules_proto.git


Covered by [BCR compatibility
test](https://buildkite.com/bazel/bcr-bazel-compatibility-test):

- https://github.com/google/glog.git
- https://github.com/google/protobuf.git
- https://github.com/google/re2.git
- https://github.com/bazelbuild/rules_apple.git
- https://github.com/bazel-contrib/rules_dotnet.git
- https://github.com/bazelbuild/rules_foreign_cc.git
- https://github.com/bazelbuild/rules_go.git
- https://github.com/sgammon/rules_graalvm.git
- https://github.com/tweag/rules_haskell.git
- https://github.com/bazelbuild/rules_jsonnet.git
- https://github.com/bazelbuild/rules_jvm_external.git
- https://github.com/bazelbuild/rules_kotlin.git
- https://github.com/bazelbuild/rules_nodejs.git
- https://github.com/bazelbuild/rules_perl.git
- https://github.com/bazelbuild/rules_python.git
- https://github.com/bazelbuild/rules_rust.git
- https://github.com/bazelbuild/rules_scala.git
- https://github.com/bazelbuild/rules_swift.git
- https://github.com/bazelbuild/rules_webtesting.git

We are being a bit aggressive on removing projects from the downstream
pipeline since we believe the BCR Bazel compatibility test will do a
good job on catching Bazel regressions and notify BCR module maintainers
about future breaking changes.

If a project maintainer wants to catch up with changes from Bazel@HEAD
more timely, it's recommended to set up test configs in your own CI with
Bazel at `last_green` (updated hourly) or `rolling` (updated ~ every two
weeks).

If a project maintainer wants to keep your project in the downstream
pipeline, please confirm you'll follow our [downstream project
policies](https://github.com/bazelbuild/continuous-integration/blob/master/docs/downstream-testing.md#downstream-project-policies).
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