Deprecation of `/`-formatted child resource names

We eventually plan to deprecate this syntax entirely in favor of the safer parent property or nested resource syntax.

Related: #6921, #2841, #2446

We may want to implement #2245 & #2246 first to have less verbose options than having to declare `existing` resources.

I've repurposed this issue to track this plan. Here's the original issue text:
> Reported under https://github.com/Azure/bicep/issues/4789.
>
> Given the following Bicep file:
> ```bicep
> param networkResourcegroup string
> param subnetName string
> param sqlSAPW string
>
> var location = resourceGroup().location
>
> resource SAPVnetSubnet 'Microsoft.Network/virtualNetworks/subnets@2021-02-01' existing = {
> name: subnetName
> scope: resourceGroup(networkResourcegroup)
> }
>
>
> resource SAPAzSQL 'Microsoft.Sql/managedInstances@2021-02-01-preview' = {
> name: 'sql-sap-prod-001'
> location: location
> sku: {
> name: 'GP_Gen5'
> tier: 'GeneralPurpose'
> capacity: 2
> }
> properties: {
> subnetId: SAPVnetSubnet.id
> storageSizeInGB: 32
> collation: 'SQL_Latin1_General_CP1_CI_AS'
> minimalTlsVersion: 'TLS_1_2'
> zoneRedundant: false
> administratorLogin: 'azsql'
> administratorLoginPassword: sqlSAPW
> vCores: 4
> licenseType: 'BasePrice'
> publicDataEndpointEnabled: false
> storageAccountType: 'LRS'
> }
> }
> ```
>
> It's very easy to miss the fact that the `SAPVnetSubnet` expects a `name` property to be fully-qualified (including both the vnet name, and the subnet name, separated by a `/`).
>
> To make things worse, because Bicep generates the following template expression, the error is quite cryptic:
> ```
> "[extensionResourceId(
> format('/subscriptions/{0}/resourceGroups/{1}',
> subscription().subscriptionId,
> parameters('networkResourcegroup')),
> 'Microsoft.Network/virtualNetworks/subnets',
> split(parameters('subnetName'), '/')[0],
> split(parameters('subnetName'), '/')[1]
> )]"
> ```
>
> The error returned (reference to an array index when there is no array index usage in the original Bicep file):
> ```
> The language expression property array index '1' is out of bounds.
> ```