Output release version to caller workflow

Some projects output both Scala and NPM libraries with a single
release - for example, we need to support this in content-api-models:

guardian/content-api-models#232

...rather than try to teach gha-scala-library-release-workflow how to
do NPM releases, it seems better to let the workflow do the Scala
release, and then output the release version number so that it
can be used by a subsequent custom 'npm release' stage of caller
workflow.

See also: https://docs.github.com/en/actions/using-workflows/reusing-workflows#using-outputs-from-a-reusable-workflow

.github/workflows/reusable-release.yml

@@ -28,6 +28,10 @@ on:
           Should be in normal plaintext 'BEGIN PGP PUBLIC KEY BLOCK' (ASCII-armored) format, with no additional BASE64-encoding.
           The passphrase can be removed from an existing key using 'gpg --edit-key <key-id> passwd' : https://unix.stackexchange.com/a/550538/46453"
         required: true
+    outputs:
+      RELEASE_VERSION:
+        description: "The un-prefixed version number of the release, eg '3.0.1'"
+        value: ${{ jobs.push-release-commit.outputs.release_version }}
 
 env:
   LOCAL_ARTIFACTS_STAGING_PATH: /tmp/artifact_staging