Default enable hash_oauth_secrets

What does this MR do and why?

Related to #374588 (closed)

We've been using hashed OAuth Tokens on GitLab.com for a day without any issues. It has been enabled in staging without any issues for 3 months. This will default-enable the flag for all self-managed instances, as well. After enabling this flag, users will lose the ability to copy application secrets other than immediately after creating the application. Inn the event anything goes wrong, self-managed customers can disable the flag by running Feature.disable(:hash_oauth_secrets) in the Rails console.

Enable the feature flag hashed_oauth_secrets by default

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