Create a supporting branch for the current major + SF

[elia]

• Blocked by Merge "low-hanging PRs" and release one last minor for v4.x #143
• Create a v4 branch and mark it as a protected branch, similarly to what we did with solidus_paypal_commerce_platform

As stated in #135 we're about to reboot this extension with only SSF support. This means that if we need to release patch or minor level updates to the current version (v4.x) we need a branch from which we can release them.