Update admin-menu reducer to be keyed by siteID

[getdave]

This is a Sub Issue of #45435

Depends on #45495.

Once D49005-code is transformed to be a site-specific endpoint (@obenland are you able to help here?) we need to update the reducer introduced in #45495 so that the data is keyed by siteId.

The fetch query should return this information from the API endpoint once it is updated and then will need to be fed back through the data-layer handler function into the action in order that siteId is available in the reducer.

Once in place it's a matter of updating the reducer to use the keyedReducer helper - it will look similar to something like this:

export default keyedReducer( 'siteId', adminMenu );

[getdave]

This ended up being handled in #45495