Remove ci_restrict_cookie_auth_linting feature flag

Avielle Wolferequested to merge
565424-remove-restrict-cookie-auth-ff into master

What does this MR do and why?

Do not merge before Monday Sep 8, 2025

READY TO MERGE

The feature flag has been fully rolled out and is no longer needed. CI config linting now always requires API authentication.

Changelog: changed

Rollout issue: Rollout ci_restrict_cookie_auth_linting feature... (#565424 - closed)

Edited by Avielle Wolfe

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