Deprecate the master-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: I26d1caa249e2a1d82b4f2c610b837467a517ba47
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/80068
Reviewed-by: Adam Langley <agl@google.com>