"/admin" is an undocumented reserved word for function proxies

I know that the Azure Function App proxies function is deprecated in v4 function apps. However, for v3 apps, if you create a function app proxy with the path of "/admin", it will not proxy the request and return a 404 error without any additional information. If you change the proxy to something other than "/admin", it will work as expected. As far as I can tell, this behavior is undocumented.

```
{
"$schema": "http://json.schemastore.org/proxies",
"proxies": {
"ProxyLocation": {
"desc": [
"ProxyLocation"
],
"debug": false,
"disabled": false,
"matchCondition": {
"methods": [
"HEAD",
"OPTIONS",
"GET",
"POST",
"PUT",
"PATCH",
"DELETE"
],
"route": "/admin/{*path}"
},
"backendUri": "https://example.com/{path}"
}
}
```