Simplify IceGrid instance name computation

[externl]

We currently have a complicated process for determining the IceGrid instance name:

    // The IceGrid instance name. We can't use getIceProperty as we don't want to get any default values.
    string instanceName = properties->getProperty("IceGrid.InstanceName");
    if (instanceName.empty())
    {
        instanceName = properties->getProperty("IceLocatorDiscovery.InstanceName");
    }
    if (instanceName.empty())
    {
        instanceName = communicator()->getDefaultLocator()->ice_getIdentity().category;
    }
    if (instanceName.empty())
    {
        instanceName = "IceGrid";
    }

I've yet to find anywhere that we document this in the docs. I think we should just use IceGrid.InstanceName

[bernardnormier]

We rely on the Ice.Default.Locator -derived instance name for the config files we ship, so we can't change this logic.

https://github.com/zeroc-ice/ice/blob/main/cpp/config/icegridnode.cfg

We should however fix the doc.