TrigAnalysisTest: Switch 21.0-mc16a/d tests to use CC7 builds
Switch the "trigRel21" tests running RDOtoRDOTrigger in the 21.0-mc16a/d nightlies to use CC7 builds instead of SLC6.
Requested by @jchapman in ATLINFR-3850
MR submitted from GitLab Web IDE