Refactor SSO enforcement related code

What does this MR do and why?

Related to #405021 (closed), !115254 (comment 1343154411)

This MR is the last among others !117389 (merged), !117582 (merged), !118400 (merged), !118562 (merged), !118596 (merged) to prevent spreading SSO enforcement logic in the codebase. This MR extracts the remaining code to Gitlab::Auth::GroupSaml::SsoEnforcer class.

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