Run build on zulu distribution of JDKs 17 and 21 (#259)

Switch to zulu distribution and use 21 A
(b/c temurin does not yet have 21 GA)

.github/workflows/build.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        java_version: [ '17', '20' ]
+        java_version: [ '17', '21' ]
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -26,7 +26,7 @@ jobs:
         uses: actions/setup-java@v3.13.0
         with:
           java-version: ${{ matrix.java_version }}
-          distribution: 'temurin'
+          distribution: 'zulu'
           check-latest: true
 
       # Cache all the things