feat(notifications): migrate to backstage ui

migrated notifications plugin to use backstage ui instead
material ui.

Signed-off-by: Hellgren Heikki <heikki.hellgren@op.fi>

plugins/notifications/package.json

@@ -59,11 +59,8 @@
     "@backstage/plugin-signals-react": "workspace:^",
     "@backstage/theme": "workspace:^",
     "@backstage/ui": "workspace:^",
-    "@material-ui/core": "^4.9.13",
-    "@material-ui/icons": "^4.9.1",
     "@remixicon/react": "^4.6.0",
     "lodash": "^4.17.21",
-    "material-ui-confirm": "^3.0.12",
     "notistack": "^3.0.1",
     "react-relative-time": "^0.0.9",
     "react-use": "^17.2.4"