Backport of 'Gate trial CTA's using redirect based on FF' into 18.11

What does this MR do and why?

Backport of !235255 (merged) to 18-11-stable-ee for inclusion in the 18.11.2 patch release.

Reverts the CTA link changes from !232188 (merged) in favor of redirecting at the route layer, so all navigation sites (including the non-SM paths that are also rendered in SM cases) are gated by the automatic_self_managed_trial_activation feature flag. The CDot endpoint the in-app trial flow uses will be disabled, so the redirect must cover every entry point.

Required for 18.11.2 because: !232419 (merged) (the earlier 18.11.1 backport of !232188 (merged)) only gated the SM-specific CTAs and left the non-SM paths un-gated. The CDot endpoint disablement would otherwise still break the SM trial flow via those un-gated entry points.

Original MR: !235255 (merged) (merged to master 2026-05-14) Squash commit picked: d65ef8b06cd23385a0159423ac6c4386113059f7

MR acceptance checklist

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
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• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, reach out to a Software Engineer in Test in #s_developer_experience.

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