CirrusSearch used to pass custom Groovy scripts to ES dynamically, but they've been changing that for security reasons and script.disable_dynamic will be restored to true (which is usually the default value)
Flow\Search\Searcher::termsAggregation is an aggregation script that relies on script.disable_dynamic: false.

AIUI, in CirrusSearch, all Groovy scripts are also being converted to Java. I think we could still use Groovy if we'd really want to, but our script is trivial enough to convert easily.