56367: roachtest: add 20.2 fixtures for 21.1 mixed-version tests r=lucy-zhang a=lucy-zhang

This commit adds a 21.1 entry to `PredecessorVersion` (currently at
`20.2.0-rc.3`, but we can update it soon) and adds fixtures for the
`acceptance/mixed-version` roachtest.

Release note: None

Co-authored-by: Lucy Zhang <[email protected]>