Go back to JDK 11 for mutation tests

When running the mutation tests in JDK 17, they seem to incorrectly report lower
coverage than what is actually the case - including whole classes plummeting to
0% coverage. I have manually confirmed that some of the mutations reported as
uncovered are in fact covered by the tests. Perhaps there's some test discovery
mechanism that breaks in JDK 17; I'll have to test newer versions of the test
frameworks and see if that helps.

Author: emlun

.github/workflows/coverage.yml

@@ -18,10 +18,10 @@ jobs:
     - name: Check out code
       uses: actions/checkout@v3
 
-    - name: Set up JDK 11
+    - name: Set up JDK
       uses: actions/setup-java@v3
       with:
-        java-version: 17
+        java-version: 11
         distribution: temurin
 
     - name: Run mutation test