* Automatically derive active protos target used in proto_format.sh from the packages with files
  annotated as active.

* Replace some sites with hardcoded v2/v3 with active/next_major_version_candidate.

* Move //docs:BUILD to //versioning:BUILD in API tree. This has already ceased to be used directly
  in the docs build, so this makes things a bit clearer.

Risk level: Low (API tooling and build only)
Testing: fix_format (mostly idempotent)

Part of #10355.

Signed-off-by: Harvey Tuch <htuch@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 4403003b9c0888ce81606a2f45fcc1da126514e7