Move header_read_timeout_buffered_io feature flag

What does this MR do and why?

In !78065 (merged) we added a timeout for reading header data. We would use a patched Net::HTTP class, which then uses a patched Net::BufferedIO class. This is a risky change because it is overriding two ruby core libraries (Net::HTTP and Net::BufferedIO). To limit the impact, we added two security measures:

• The patched classes are only used if the use_read_total_timeout option is used for the Gitlab::HTTP request.
• A timeout error is only raised when the header_read_timeout_buffered_io feature is active.

In this MR we are only using the patched classes when the header_read_timeout_buffered_io feature is active and removing the check for the use_read_total_timeout option because:

• Using the feature flag to control usage of the patched classes lets us better control the rollout. In case something is wrong with classes, we can simply turn it off.
• The use_read_total_timeout option was actually intended to control the ReadTotalTimeout and we want to remove the option in a future iteration
• An option to opt-out of the timeout is not needed because reading header data is never expected to run into a timeout.

Screenshots or screen recordings

I've recorded a video that explains read total timeout. It is not showing the feature that is touched in this MR but it might be helpful to watch: 📺 https://www.youtube.com/watch?v=SqJGXMf-5RU (The video is private for now because the related issue is still confidential)

How to set up and validate locally

See #336999 (comment 808312938) (confidential)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.