Includes cleanup to simplify our setup. Major changes:
- Eliminate the NNBD plugin filtering for stable.
- Remove the temporarily-added beta branch testing.
- Enable Linux, macOS, and web on stable (Windows is LUCI-based)
- Combine the two different macOS matrix configurations now that they
are the same.
- Combine the two different Linux matrix configurations by using a single
Dockerfile (which now also includes clang-format)
- The web integration smoke test temporarily still uses the old Dockerfile,
now renamed, because the driver installer script doesn't support
Chrome 89 yet.
- Move most of the Linux tasks to lower-CPU machines to allow more
tasks to run in parallel without hitting the community limit.
- Reorder the tasks slightly and give them comments to identify
platform groupings
- Enabled web "build all plugins together" and "build all examples"
tests
