JDK version bump cleaups, bump minimum Gradle version to 8.1 (#454)

* Bump JDK when running simple gradle CI jobs
* Bump minimum Gradle version to 8.1
* Fix docs, and fix minimum version checker

pkl-cli/pkl-cli.gradle.kts

@@ -321,7 +321,7 @@ publishing {
         description.set("""
           Pkl CLI executable for Java.
           Can be executed directly on *nix (if the `java` command is found on the PATH) and with `java -jar` otherwise.
-          Requires Java 11 or higher.
+          Requires Java 17 or higher.
         """.trimIndent())
       }
     }