Default enable header_read_timeout_buffered_io feature flag

What does this MR do and why?

Rollout issue: #350233 (closed)

This default enables the header_read_timeout_buffered_io feature flag. It is already enabled for 100% on production (#350233 (comment 858749357)).

Screenshots or screen recordings

See the original MR screen recording

How to set up and validate locally

See #336999 (comment 808312938) (confidential)

MR acceptance checklist

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