Clean up old Gitlab::HTTP related codes

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After closing #415686 (closed) and #426549 (closed), we need to remove old Gitlab::HTTP related codes from the codebase.

• Check Gitlab::HTTPConnectionAdapter and its usage places.
• Check Gitlab::NetHttpAdapter and its usage places.
• Check Gitlab::UrlBlocker and its usage places.
• Check Gitlab::UrlBlockers::UrlAllowlist and its usage places.
• Check Gitlab::UrlBlockers::DomainAllowlistEntry and its usage places.
• Check Gitlab::UrlBlockers::IpAllowlistEntry and its usage places.
• Check Gitlab::BufferedIo and its usage places.
• Fix the usage of deny_all_requests_except_allowed because it will never be a Proc.
• Gitlab::CurrentSettings.network_settings; !143050 (comment 1755064637)
• Fix the comment in #440772
• !148368 (comment 1843748064) "Maybe this could be extracted as a separate class to be reused in the other service? so that we clump together the options and logic so that they stay in sync."