Admin Update Breaks Access - Missing dashboard.html.twig

On 1.8.x (where I have to continue working for the time being), the latest Admin update rendered it unusable. I get the following error on login:

```
RuntimeException (400)
Template "partials/dashboard-migration-notice.html.twig" is not defined in "dashboard.html.twig" at line 38.
```

Per GLM5, the issue appears to be that commit a31579d2 ("Grav plugin updates") added an include for dashboard-migration-notice.html.twig but the file was never created.

Inspect dashboard.html.twig around line 38 and the include for partials/dashboard-migration-notice.html.twig, then review commit a31579d2 to understand the Admin update. Reproduce the login failure on the 1.8.x branch and verify that the dashboard renders without the missing-template RuntimeException.

Written by the indexing model from the issue text.