Add require_personal_access_token_expiry application setting

What does this MR do and why?

This backports !158186 (merged) to 17-1-stable-ee for gitlab-org/release/tasks#11510 (closed).

Adds a new admin setting, ApplicationSettings#require_personal_access_token_expiry that removes the expiration enforcement of PAT, PrAT and GrATs A SM admins can optionally enable the setting Require access tokens to have an expiry to require expiration for their tokens. This will only be applicable to net new tokens.

Related to #470192 (closed).

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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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