update OsmoNITB migration guide
OsmoNITB_Migration_Guide is showing its age. I've fixed the two most obvious outdated bits (no subscriber-create-on-demand and references to bsc_mgcp), but I'm sure there are plenty more, particularly in the config file snippets.
As there may still be people migrating from OsmoNITB, let's update it to reflect the current situation.
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