add 12.x to abi_migration_branches

I had considered switching patterns for 10.x and 11.x already;
it was a mistake not to do it, so do it now. Better do the right
thing long-term; it'll only take a little while until the old
pattern falls off the radar.

conda-forge.yml

@@ -3,6 +3,7 @@ azure:
   max_parallel: 20
 bot:
   abi_migration_branches:
+  - 12.x
   - 11.0.x
   - 10.0.x
   - 9.0.x