Change default mvd to v20a_tr

Following https://redmine.cbm.gsi.de/issues/2119 and SWM discussion, recommended MVD version "v20c_tr" was not compatiable with sts "v19a".
This change should be reverted once a new STS geometry is available.
3 jobs for !87 with setup_mvd in 41 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Check Repository
passed LinearHistCheck #26492
docker

00:00:22

passed RebaseCheck #26491
docker

00:00:23

 
  Check Format
passed FileFormatCheck #26493
docker

00:00:17