Skip preloading Requests files in `_preload_old_requests_classes_and_interfaces()` when updating from a WordPress version older than 4.6.
Why?
Requests library was first introduced into WordPress 4.6 via #33055 / [37428]. If a user is upgrading from a version older than 4.6, this changeset prevents the Requests preloading to prevent a fatal error of attempting to load files that do not exist in their current WordPress version.
Follow-up to [54997], [37428].
Props afragen, costdev, ironprogrammer, antonvlasenko.
Fixes#57662.
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