Change MetaMetrics category for background events (#9155)

Background events are now sent in the `Background` category, rather
than `backend`. Conventionally we use the term "background" over
"backend", as it's not really a "backend" in the normal sense since
it's a client background process. Also it's capitalized because all of
the other event categories are capitalized as well.

The metrics URL has been updated to use `background` instead of
`backend` as well, for consistency.

Luckily we don't have to worry about our metrics being disjointed due
to this name change, because the background metrics never worked to
begin with! So there will be none under the old name. The metrics will
be made functional in a separate PR.

app/scripts/lib/backend-metametrics.js

@@ -14,7 +14,7 @@ export default function backEndMetaMetricsEvent (metaMaskState, eventData) {
     sendMetaMetricsEvent({
       ...stateEventData,
       ...eventData,
-      url: METAMETRICS_TRACKING_URL + '/backend',
+      url: METAMETRICS_TRACKING_URL + '/background',
     })
   }
 }

app/scripts/metamask-controller.js

@@ -257,7 +257,7 @@ export default class MetamaskController extends EventEmitter {
               errorMessage: txMeta.simulationFails?.reason,
             },
             eventOpts: {
-              category: 'backend',
+              category: 'Background',
               action: 'Transactions',
               name: 'On Chain Failure',
             },