Cleanup invisible_captcha feature flag

The invisible_captcha feature flag has been in use since 12.2. A decision has been made to convert it into an application setting (similar to the recaptcha_enabled application setting).

Proposal is to:

1. make it a boolean and name it invisible_captcha_enabled.
2. add relevant documentation.

After converting, the application setting should be enabled on production (https://gitlab.com/gitlab-com/gl-infra/production/-/issues/3313) and this command should be run in chatops: /chatops run feature delete invisible_captcha and this MR should be reverted: !48042 (merged)