TrigAnalysisTest: Switch 21.0-mc16a/d tests to use CC7 builds

Rafal Bielskirequested to merge
rbielski-master-patch-08735 into master

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

Edited by Rafal Bielski

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