fix: translate German MLS migration message [WPB-25947]

[Garzas]

https://wearezeta.atlassian.net/browse/WPB-25947

---

PR Submission Checklist for internal contributors

• The PR Title

  • conforms to the style of semantic commits messages¹ supported in Wire's Github Workflow²
  • contains a reference JIRA issue number like SQPIT-764
  • answers the question: If merged, this PR will: ... ³

• The PR Description

  • is free of optional paragraphs and you have filled the relevant parts to the best of your ability

---

What's new in this PR?

• Add German translation for the MLS migration start system message.
• Add MLS-specific “learn more” link text and show the MLS support link for migration start messages.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

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

See analysis details on SonarQube Cloud