Re-factor repository workflow

Addresses #13

See merge request !18

Also rolled back ACTS/Gaudi/python to their v2.3.x versions as this is the last version that works with our downstream software. We can re-upgrade after we set this stable baseline with the new CI implementation.

New feature version

See merge request !16

Major updates to the CI, now avoid repeat pipeline definitions by passing minimal artifacts between jobs

Streamlining gitlab CI, removed testing/staging step from workflow and added CI rules for merge requests

	modified:   VERSION

Updated container and software versions.

See merge request !15