Add CI support for upstream RT stable
## Issue description In order to help with upstream maintenance, and in turn improve the quality of the code going into the CIP RT kernels, we want to start CI build/testing the RT project's code. ## Useful links - [linux-rt-devel](https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/) - [linux-stable-rt](https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-stable-rt.git/) ## Acceptance criteria TODO
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