Drop Scala 2 in runner, test-runner and tasty-lib modules

[Gedochao]

• this is a follow-up to Update runner & test-runner to Scala 3.3.5 LTS (was 3.0.2) #3650
• even though it was mentioned in Scala CLI v1.8.0 release notes that we'd stop, we kept releasing runner and test-runner modules for Scala 2.12 and 2.13 (even though we did bump Scala 3 version used to the 3.3 LTS)
• with this change, we are effectively freezing these 2 modules at 1.9.1; no further development is predicted to happen there, as no Scala 2.14 version is predicted to be coming
• using the runners with Scala 2 will keep on working, of course, but the newest developments and improvements will no longer be available for those versions (not that much development is happening there); future Scala CLI releases will fall back to 1.9.1 for those modules.
• an appropriate warning will get printed when the runners are used with Scala 2
  • I might do a follow-up with a a flag/config for silencing the warning (out of scope for this PR)