Align Microsoft.Network/privateEndpoints API version across all modules #1054

Description

Update Microsoft.Network/privateEndpoints and Microsoft.Network/privateEndpoints/privateDnsZoneGroups API version to to 2021-05-01 across all modules.
Currently there is a mix between 2021-05-01 and 2021-03-01 and the latter fails the static validation

See e.g. https://github.com/Azure/ResourceModules/runs/5350183177?check_suite_focus=true#step:3:594