Update Lambda runtime and CI to Java 21

As suggested in #209 (comment),
we want the Lambda runtime to match the Java version used by developers.

I've also updated CI to use Java 21. Ideally, we wouldn't have to do this separately
anymore, as setup-java has added partial support for asdf's .tool-versions file with
actions/setup-java#606, but unfortunately
actions/setup-java#615 means that it won't work with
Corretto version numbers.

.github/workflows/ci.yml

@@ -24,10 +24,10 @@ jobs:
 
       # Configuring caching is also recommended.
       # See https://github.com/actions/setup-java
-      - name: Setup Java 11
-        uses: actions/setup-java@v3
+      - name: Setup Java
+        uses: actions/setup-java@v4
         with:
-          java-version: '11'
+          java-version: '21'
           distribution: 'corretto'
           cache: 'sbt'
 

cloudformation/cfn.yaml

@@ -61,7 +61,7 @@ Resources:
               - s3:GetObject
             Resource: !Sub arn:aws:s3:::${ConfigBucket}/*
       Handler: com.gu.anghammarad.Lambda::handleRequest
-      Runtime: java11
+      Runtime: java21
       MemorySize: 512
       Timeout: 30
       CodeUri: