Removes `testenv` annotations and related code. These annotations originally let you skip x-pack snippet tests in the docs. However, that's no longer possible.
Relates to #79309, #31619
This replaces the `script` docs for bucket aggregations with runtime
fields. We expect runtime fields to be nicer to work with because you
can also fetch them or filter on them. We expect them to be faster
because their don't need this sort of `instanceof` tree:
a92a647b9f/server/src/main/java/org/elasticsearch/search/aggregations/support/values/ScriptDoubleValues.java (L42)
Relates to #69291
Co-authored-by: James Rodewig <40268737+jrodewig@users.noreply.github.com>
Co-authored-by: Adam Locke <adam.locke@elastic.co>
