* added dendrite captcha options

* added hcaptcha doc

* proper url

* Apply suggestions from code review

Co-authored-by: Slavi Pantaleev <slavi@devture.com>

* Update main.yml

* renamed captcha vars to new naming scheme

* change vars to new format

* Rename back some incorrect renamed variables

These variables are either not just part of the `client_api` subsection,
or are not even part of that section at all. They shouldn't have been
renamed in baaef2ed616e2645550d9

* Fix up naming inconsistencies

Some of these variables had been renamed in one place,
but not in other places, so it couldn't have worked that way.

* Add validation/deprecation for renamed Dendrite variables

Related to 4097898f885cf4c73, baaef2ed616e2645550, 68f4418092fa8ad
and a0b4a0ae6b2f1f18

Co-authored-by: Slavi Pantaleev <slavi@devture.com>