Fix admin-menu so that data layer only accepts numeric values for siteID

[getdave]

Sub Issue of #45435.

Currently as of #45495 it is possible to make a request like this:

dispatch({ 
    type: 'ADMIN_MENU_REQUEST', 
    siteId: 123456,
});
dispatch({ 
    type: 'ADMIN_MENU_REQUEST', 
    siteId: '{{DOMAIN_OF_YOUR_WPCOM_TESTING_SITE}}'
});

...and the resulting state will be:

{
    123456: { // menu data here }
    '{{DOMAIN_OF_YOUR_WPCOM_TESTING_SITE}}': { // menu data here }
}

So basically the state ends up being "keyed" by different types of data:

1. Int
2. String

Ideally, we should be consistent and use one or the other. We need to adjust the data layer to only allow requests using numeric values for siteId.

[github-actions]

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