Linter: update to eslint v9

This eslint version brings breaking changes related to the eslint config.
eslint-airbnb-config has yet to release a compatible version with the
new format, and it generally looks unmaintained, hence we drop the dependency
in favor of the built-in recommended configs.

test/worker/worker_example.js

@@ -1,4 +1,4 @@
-/* globals openpgp */
+/* globals openpgp, importScripts */
 
 importScripts('../../dist/openpgp.js');