Changing the provisioning password hashing algorithm

[Lennonka]

https://theforeman-foreman-documentation-preview-pr-3922.surge.sh/nightly/Provisioning_Hosts/index-foreman-el.html#Changing_the_Provisioning_Password_Hashing_Algorithm_provisioning

I think we should mention that SHA512 is recommended since EL5 and became default in EL6. Lowering it to SHA256 will lower security, not raise it. Since Foreman 3.12 the default changed from SHA256 to SHA512. Note it didn't update existing operating systems on upgrade.

Quoting theforeman/foreman@e2dee7d

SHA256 is no longer considered supported and is prohibitted (sic) by some policies

So I think we should drop this chapter.

Originally posted by @ekohl in #3922 (review)