add missing capabilities checks (#117072)

This should fix a couple of tests failures due to incorrect capabilities checks. I already added them manually to 8.x to resolve a failure there (see #116602) so this doesn't need to be backported.

Author: not-napoleon

x-pack/plugin/esql/qa/testFixtures/src/main/resources/union_types.csv-spec

@@ -734,6 +734,8 @@ count:long  |  @timestamp:date
 Multi Index millis to nanos stats
 required_capability: union_types
 required_capability: union_types_remove_fields
+required_capability: to_date_nanos
+required_capability: date_trunc_date_nanos
 
 FROM sample_data, sample_data_ts_nanos
 | EVAL @timestamp = DATE_TRUNC(1 hour, TO_DATE_NANOS(@timestamp))