chore: Keycloak Migration

[cmaddox5]

Summary of changes

Asana Ticket: migrate signs-ui from Cognito to Keycloak

This PR converts auth from Cognito to Keycloak. Most changes were made following this guide. I also removed all references to the RefreshTokenStore. All usages of the refresh token were removed back in #734 so the store saves the tokens but nothing actually uses them.

Reviewer Checklist

• Meets ticket's acceptance criteria
• Any new or changed functions have typespecs
• Tests were added for any new functionality (don't just rely on coverage statistics)
• signs.json changes were also made in realtime_signs
• Ask product if custom label updates in mbta.ts should also be made to paess_labels.json in Screenplay
• This branch was deployed to the staging environment and is currently running with no unexpected increase in warnings, and no errors or crashes (compare on Splunk: staging vs. prod)