MS-1311 Remove deprecations from Firebase Auth manager

[luhmirin-s]

JIRA ticket
Will be released in: 2026.2.0

Notable changes

• Removing deprecated Firebase app handling since there are no more sessions from such old app versions in the dashboard

Testing guidance

• Regular regression testing is fine

Additional work checklist

• Effect on other features and security has been considered
• Design document marked as "In development" (if applicable)
• External (Gitbook) and internal (Confluence) Documentation is up to date (or ticket created)
• Test cases in Testiny are up to date (or ticket created)
• Other teams notified about the changes (if applicable)

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud