add all-green job, which checks if all required jobs have succeeded; can be used for branch protection

.github/workflows/ci.yml

@@ -292,3 +292,22 @@ jobs:
 
       - name: scaladoc ${{ matrix.scala }}
         run: sbt "++${{ matrix.scala }}; Compile/doc"
+
+  # One status check to require in branch protection rules, instead of every matrix leg by name.
+  # See: https://github.com/re-actors/alls-green
+  all-green:
+    runs-on: ubuntu-latest
+    if: always()
+    needs:
+      - sbt-tasks
+      - test-coverage
+      - coverage-finished
+      - binary-compatibility
+      - formatting
+      - scaladoc
+
+    steps:
+      - name: decide whether the needed jobs succeeded or failed
+        uses: re-actors/alls-green@b5b5b37504aa4183270bd3d855c52a67f212be35 # v1.3.0
+        with:
+          jobs: ${{ toJSON(needs) }}

README.md

@@ -83,6 +83,10 @@ to improve the quality of code in repo!
 * `formatting` - runs [scalafmt](https://scalameta.org/scalafmt/)'s `scalafmtCheckRepo` task (requires at 
   least version 2.6.2)
 * `scaladoc` - calls `Compile/doc` task to make sure that Scaladocs compile
+* `all-green` - a single check that passes only if all of the above did, via
+  [alls-green](https://github.com/re-actors/alls-green). Require it in branch protection rules as
+  `test / all-green` (named after the caller's job) rather than listing every matrix leg; callers no
+  longer need their own `all-green` job
 
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