Hide JWT toggle behind a feature flag

What does this MR do and why?

For #391886 (closed)

We are hiding the setting under the ci_remove_jwt_toggle feature flag so we can control the timing of when the setting is removed. The setting will be enabled by default for all projects in 16.0.

Cleanup issue: !115644 (merged)

Rollout issue: #399108 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the ci_remove_jwt_toggle feature flag.
2. Go to Settings > CI/CD > Token Access.
3. Verify that the Limit JSON Web Token (JWT) access section is not rendered.

MR acceptance checklist

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