Deprecate the main-with-bazel branch

After some infrastructure changes, this branch has not been updated
since April.

Instead, use the `main` branch, which now itself has Bazel build.

Change-Id: I5ecefba55431d154b5189fca3e3f9cebbc68b60d
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/80087
Reviewed-by: Adam Langley <agl@google.com>