Revert "Use JDK 20 instead of 18 in GitHub Actions builds"

This reverts commit c85f0a3.

Author: emlun

.github/workflows/build.yml

@@ -19,7 +19,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        java: [8, 11, 17, 20]
+        java: [8, 11, 17, 18]
         distribution: [temurin]
         include:
           - java: 17