Adopt guardian/setup-scala to fix CI

The CI is currently broken on this repository because it expects sbt to
be preinstalled on the runners but ubuntu-latest now points to an
ubuntu-24.04 build that doesn’t include sbt (e.g. see [this
build](https://github.com/guardian/flexible-restorer/actions/runs/13160341152/job/36727220402?pr=110)).

To fix CI, this commit adopts
[guardian/setup-scala](https://github.com/guardian/setup-scala) to
install sbt on the runners and configure it how we usually do.

To use setup-scala, a .tool-versions file specifying the desired version
of Java is required: this commit adds this pointing to a Java 11 version
of corretto, which this app was already using.

.github/workflows/ci.yml

@@ -40,11 +40,7 @@ jobs:
                   npm install
                   npm run build
 
-            - uses: actions/setup-java@v3
-              with:
-                  java-version: '11'
-                  distribution: 'corretto'
-                  cache: 'sbt'
+            - uses: guardian/setup-scala@v1
 
             - name: Scala Build
               run: |

.tool-versions

@@ -0,0 +1 @@
+java corretto-11.0.25.9.1