Change feature flag in JiraConnect OAuth setting controller

What does this MR do and why?

This changes the feature flag used to control the exposure of the jira_connect_application_key setting. When GitLab.com is used as a proxy for the GitLab.com for Jira Cloud app, it needs to fetch the OAuth application ID from a self-managed instance. jira_connect_oauth_self_managed_setting feature flag will be enabled by default and rolled out to self-managed users.

This prepared self-managed users for the new feature and they will be able to use it as soon as the jira_connect_oauth_self_managed feature is enabled on GitLab.com

MR acceptance checklist

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