Add java 21 to build matrix

Author: Philippus

.github/workflows/ci.yml

@@ -23,7 +23,7 @@ jobs:
       matrix:
         os: [ubuntu-latest]
         scala: [2.12.19]
-        java: [temurin@8, temurin@11, temurin@17]
+        java: [temurin@8, temurin@11, temurin@17, temurin@21]
     runs-on: ${{ matrix.os }}
     steps:
       - name: Checkout current branch (full)
@@ -55,6 +55,14 @@ jobs:
           java-version: 17
           cache: sbt
 
+      - name: Setup Java (temurin@21)
+        if: matrix.java == 'temurin@21'
+        uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: 21
+          cache: sbt
+
       - name: Setup sbt
         uses: sbt/setup-sbt@v1