Use the API to fetch the remote default branch

A simpler and quicker way to fetch the default branch of a remote Git URL.

• Based on #1035 (comment 602953289).
• A more solid solution of !1885 (closed).
• Fixes #1037 (closed).

Test locally

1. Check that building the latest image fails:

   docker build --build-arg NANOC_ENV=production --build-arg=CI_COMMIT_REF_NAME=main -t docs:latest -f Dockerfile.main .

2. Build the base image that includes jq !1886 (merged)

   docker build -t registry.gitlab.com/gitlab-org/gitlab-docs:base -f dockerfiles/Dockerfile.gitlab-docs-base .

3. Build the bootstrap image:

   docker build -t registry.gitlab.com/gitlab-org/gitlab-docs:bootstrap -f dockerfiles/Dockerfile.bootstrap .

4. Build the latest image and see it succeeding:

   docker build --build-arg NANOC_ENV=production --build-arg=CI_COMMIT_REF_NAME=main -t docs:latest -f Dockerfile.main .

After this is merged

1. Find the latest scheduled pipeline in https://gitlab.com/gitlab-org/gitlab-docs/-/pipelines and run the image:docs-latest job manually.
2. Verify that the issue #1035 (closed) is fixed and close it.