Fixes #34236 - Drop require_ssl_smart_proxies setting

This defaults to true and setting it to false can create security
problems. Mandating client SSL certificates creates a more secure
environment.

Previously when require_ssl_smart_proxies was false, reverse DNS was...

Fixes #23300 - Brakeman SQL injections

Fix all foundings of SQL injections by
Brakeman: "brakeman --test SQL"

Fixes #34166 - opt in the new host page by default

Fixes #33417 - Hide version from the Login page

Refs #33417 - revert snapshot changes

those snapshots are not relevant to this PR

Fixes #33996 - Clean up deprecated methods

Cleans up all methods that have been marked for removal in 3.2 or older.

Fixes #34002 - add Foreman::Plugin#medium_providers_registry

Also, modify all registries on the instance to be delegated to the
class in one place instead of some doing the delegation inside a method.

Fixes #33892 - add setting validations in to_prepare (#8911)

Setting validations need to be added on code reload.
Also this ensures the Validator classes are not loaded prematurelly.

Adds a class that holds all the validations to add them on reload.

Fixes #33906 - Drop fix_db_cache setting

This setting was introduced when the current permissions model was
introduced in Foreman 1.7 to allow recreating the user permissions cache
after the migration from the previous model. The setting has no other
uses....

Fixes #33484 - add setting for opt-in/out the new host details

Fixes #32064 - drop Puppet related settings

Keeps Puppet CA settings as Puppet CA is still in.

Extracted in: theforeman/foreman_puppet#122