Migrate scala-stm benchmarks from Scala 2.12 to Scala 3

[lbulej]

Migrates the scala-stm benchmarks to Scala 3 (skipping Scala 2.13), leaving only the reactors benchmark to be migrated to at least Scala 2.13 (#324).

Below are plots from simple measurements I collected from 15 runs (150 repetitions each) using JDK 11 (default settings) on 8 cores of Xeon E5-2697A v4:

• the scala212 variant corresponds to commit 87358d9 (Renaissance 0.15.0, scala-stm 0.8 with Scala 2.12.18)
• the scala3_stm variant corresponds to commit 6d1cb1f (scala-stm 0.11.1 with Scala 3.3.1)