Run Java tests on generated schemas in stefgen on Github

We have testcases in stefgen/generator/testdata which currently generate Go and Java code from schemas, but only run Go tests. We need to also run Java tests.

The action sets up both Go and Java, builds stefgen using Go, generates Java code from schemas using stefgen, then runs Java tests.

#114

Author: tigrannajaryan

.github/workflows/test-java-schemas.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        java-version: [17, 21]
+        java-version: [17, 23]
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4