Enable automatic backporting for linux kernel packages

[Vincinator]

What would you like to be added:

Automatically start backport builds where possible.

Why is this needed:

🚧 dumped all the input into this ticket, need to clean this issue and make it more concise later 🚧

We have rel-XXX branches where XXX is the major version of a supported Garden Linux version.
This branch currently pulls in changes from a maint-X.Y or main branch where X.Y is the kernel version used, in case main is on something newer already.

Currently the workflow looks like this:

1. upgrade maint-X.Y branch with latest patch level (semi automated already, but requires manual fixing)
2. (only for first backport for XXX) create rel-XXX branch and add .container file with correct build container
3. merge maint-X.Y to rel-XXX

If Maintainer does not merge changes from main-X.Y, we can potentially diverge from the expected source in maint-X.Y, which may lead in missing fixes or patches.

To Remove this source of issue, we propose to add a definition in maint-6.6 to tell the pipelines for what versions the kernel must be build with which container (previously specified in .container).

To allow maintainers to browser git history, and to be consistent with all other packages, the rel- branches must still exist but must be updated by automation.

[Vincinator]

Automate backport merge #84 is one possible solution. need to test in a fork.