storybook: Upgrade Storybook to version 6 (#2296)

* upgrade Storybook to version 6

* update storybook renderer layout

Co-authored-by: Omer Farooq <17722640+o-farooq@users.noreply.github.com>

packages/storybook/.storybook/preview.js

@@ -22,4 +22,5 @@ addParameters({
     // Set the initial theme
     current: 'light',
   },
+  layout: 'fullscreen',
 });

packages/storybook/package.json

@@ -17,11 +17,11 @@
     "@backstage/theme": "^0.1.1-alpha.21"
   },
   "devDependencies": {
-    "@storybook/addon-actions": "^5.3.17",
-    "@storybook/addon-links": "^5.3.17",
-    "@storybook/addon-storysource": "^5.3.18",
-    "@storybook/addons": "^6.0.4",
-    "@storybook/react": "^5.3.17",
-    "storybook-dark-mode": "^0.6.1"
+    "@storybook/addon-actions": "^6.0.21",
+    "@storybook/addon-links": "^6.0.21",
+    "@storybook/addon-storysource": "^6.0.21",
+    "@storybook/addons": "^6.0.21",
+    "@storybook/react": "^6.0.21",
+    "storybook-dark-mode": "^1.0.2"
   }
 }