chore: ci: Bump devtools image and clang-format to RHEL8

This MR changes the CI configuration file .gitlab.ci-yml to run code quality checks with the RHEL8 (ubi8)-based image of scouting-devtools.

Notably, the change allows us to move to a more up-to-date clang-format version in the CI, which will be compatible with most local machines.

The registry address will be changed before merging, to match the tagged version in scouting-demonstrator/scouting-docker-images.

Partially fixes issue #56 (closed) (SCDAQ itself has yet to be moved to RHEL8)