Explicitly setup a kotlin version

Author: aeisenberg

.github/actions/prepare-test/action.yml

@@ -58,3 +58,7 @@ runs:
           echo "::error::Unrecognized version specified!"
           exit 1
         fi
+
+    - uses: fwilhe2/setup-kotlin@fa932efcc943c56bfd69ddace2e61129eb495d58
+      with:
+        version: 1.7.21