Update build.yml

Testing for Java 21

.github/workflows/build.yml

@@ -24,6 +24,7 @@ jobs:
           # For now, using the JDK that comes on the GHA runner
           cat << EOF > ~/.savant/plugins/org.savantbuild.plugin.java.properties
           17=${JAVA_HOME}
+          21=${JAVA_HOME}
           EOF
       # Takes some time, better to tell difference between this step and actual test run time
       - name: compile/pull Savant dependencies