Set the bazel language level to 11 for the plugin

We still use `--release 8` for most libraries, so in reality all this
does it make our life in IJ a little nicer: we can't break the
compatibility of the client bindings with this change.

Author: shs96c

scripts/ij.bazelproject

@@ -26,3 +26,5 @@ additional_languages:
   # scala
   typescript
 
+# We enforce the correct language level via our .bazelrc
+java_language_level: 11