release step: bump Scala 2 version in dotty repo

Author: SethTisue

Diff for: .github/ISSUE_TEMPLATE/release.md

@@ -177,6 +177,8 @@ If there are delays downstream, at some point it may make sense to go ahead and
 
 ### Afterwards
 
+- [ ] Scala 3: open PR updating version in `project/Build.scala`
+  - https://github.com/lampepfl/dotty/pulls
 - [ ] Scastie: open PR adding new version (modeled on https://github.com/scalacenter/scastie/pull/538)
   - note that the PR won't be mergeable until kind-projector has published; and if kind-projector's version number has changed, `ScalaTarget.scala` will need updating
 - ~If it's a major release:~