Specify Java 21 using asdf's .tool-versions

This is motivated by guardian/gha-scala-library-release-workflow#31 -
initially I was just thinking about updating to Java 21, but now I think
that making projects declare which Java version they use to build is
probably a good thing!

Note that we're using asdf's .tool-versions file here, which has recently gained
support in actions/setup-java#606 - this means that
we can also get _all_ our GitHub Actions files to respect the Java version
definied in that file - eg, we can reference it in ci.yml, and we don't need
to update Java version in there anymore.

.github/workflows/ci.yml

@@ -14,12 +14,12 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Setup JDK
-        uses: actions/setup-java@v3
+        uses: actions/setup-java@v4
         with:
           distribution: corretto
-          java-version: 17 # Currently Java 17 required by `s3mock-testcontainers`
+          java-version-file: .tool-versions
           cache: sbt
       - name: Build and Test
         run: sbt -v +test

.tool-versions

@@ -0,0 +1 @@
+java corretto-21.0.2.13.1