clean up cross-publishing

Right now, publishing does a `+publish` and then separate explicit `publish` invocations for 2.13 and 3.3 versions of codegen.

IIRC this is because in earlier versions of SBT, there was a problem with the sbt plugin (which is only published for 2.12) depending on codegen (when it's configured to be published for 2.13 and 3.3 as well).

It'd be great to figure out if that sbt limitation still exists. If so, try and remove it. If not, cross-publish regularly.

Start by inspecting the repository's publishing configuration and the sbt plugin and codegen cross-version settings. Check whether the earlier sbt dependency limitation still applies to Scala 2.12, 2.13, and 3.3. Done means either removing the limitation or making codegen publish through the regular cross-publishing flow.

Written by the indexing model from the issue text.