Compile akka-actor and akka-actor-tests with Scala 3 in PR validation (#29992)

.travis.yml

@@ -35,12 +35,18 @@ jobs:
     - stage: whitesource
       name: whitesource
       script: git branch -f "$TRAVIS_BRANCH" && git checkout "$TRAVIS_BRANCH" && sbt whitesourceCheckPolicies whitesourceUpdate
+    - stage: scala3
+      name: scala3
+      # separate job since only a few modules compile with Scala 3 yet
+      script: jabba install adopt@1.11-0 && jabba use adopt@1.11-0 && sbt -Dakka.build.scalaVersion=3.0 "akka-actor-tests/test:compile"
 
 stages:
   - name: whitesource
     if: repo = akka/akka AND ((branch = master AND type != pull_request) OR tag =~ ^v)
   - name: test
     if: type == pull_request OR NOT tag =~ ^v
+  - name: scala3
+    if: type == pull_request OR NOT tag =~ ^v
 
 env:
   global: