bgpd: Deprecate BGP `internet` community

bgpd: Deprecate BGP `internet` community

Quite a few well-known communities from IANA's list do
   not receive special treatment in Cisco IOS XR, and at least one
   community on Cisco IOS XR's special treatment list, internet == 0:0,
   is not formally a well-known community as it is not in [IANA-WKC] (it
   is taken from the Reserved range [0x00000000-0x0000FFFF]).

https://datatracker.ietf.org/doc/html/rfc8642

This is Cisco-specific command which is causing lots of questions when it
comes to debugging and/or configuring it properly, but overall, this behavior
is very odd and it's not clear how it should be treated between different
vendor implementations.

Let's deprecate it and let the operators use 0:0/0 communities as they want.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>