Use Oauth sign in button in Jira Connect header

What does this MR do and why?

⚠ this is behind the :jira_connect_oauth feature flag.

Follow-up to #355048 (comment 974232897).

This MR updates the sign in link in the Jira Connect app header to use the Oauth sign in button (when the feature flag is enabled).

Screenshots or screen recordings

Kapture_2022-06-10_at_17.09.07

How to set up and validate locally

Set up the Jira Connect app with Gitpod, enable the jira_connect_oauth FF and observe the rendered button: https://docs.gitlab.com/ee/development/integrations/jira_connect.html

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.