GitHub default branch is now "main"

[kptdobe]

Centralising the work here.

GitHub has now made the shift, new repositories default branch is now main. Which means that to use helix-pages after you created a new repo, you must specify the main branch in the url, master still being the default.

Some work has been done is this direction, but it is not done:

• helix-resolve-git-ref can now fallback to the default branch: fallback to default branch helix-resolve-git-ref#253
• VCL code must be changed to not default to master: do not assume default ref to be master helix-publish#617
• for testing purposes, we now default the branch to master in the VCL extensions. This would need to be aligned too. See https://github.com/adobe/helix-pages/blob/master/vcl/extensions.vcl#L100
• a default to master is done here: Do not assume default ref to be master helix-shared#376 (no sure where this is used)
• content-proxy should be able to handle a null ref: content-proxy requires ref helix-content-proxy#200
• helix-static should be able to handle a null ref: static should not fallback to master branch helix-static#415

[kptdobe]

No time at the moment to look at this one.