REST API: Change “ipv4” types to “ip” to support ipv6.

Stop presuming IP address are IPv4, instead make the type “ip” to be agnostic of IP version. This fixes requests with ipv6 addresses for comments in core.

Props dd32, schlessera, danielbachhuber.
Fixes #38818.
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git-svn-id: https://core.svn.wordpress.org/trunk@39236 1a063a9b-81f0-0310-95a4-ce76da25c4cd