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
Changes:
* Renames 'full copy searchable snapshot' to 'fully mounted index.'
* Renames 'shared cache searchable snapshot' to 'partially mounted index.'
* Removes some unneeded cache setup instructions for the frozen tier. We added a default cache size with #71844.