Deprecate /api/v0/dns

[lidel]

Problem

• /api/v0/dns does not work with DNSLinks that have /ipns/.. content paths (/api/v0/dns/ fails with "not a valid domain name" error on IPNS DNSLinks #6454), and /api/v0/resolve works fine will all content paths.
  • IPFS companion switched its DNSLink detection to /api/v0/resolve a while ago. It is passing DNSLink names as content paths: /ipns/example.com and they work fine no matter if IPNS is used or not.
• Having two ways of resolving DNSLink is confusing enough, having one that fails when content path is /ipns/ is simply harmful, and even if we fix that, adds unnecessary complexity to the API surface.
  • In reality, users just want to resolve a content path. They dont care if DNS or IPNS is involved. What matters is resolving content path. Either one level, or all the way, until immutable /ipfs/{cid} is hit.

Proposed solution

Deprecate /api/v0/dns:

• remove it from cli --help and hide from API docs
• to ensure nothng breaks, keep /api/v0/dns endpoint, but remove its custom DNS lookup code, and switch it to use /api/v0/resolve internally

[aschmahmann]

@lidel do we have any utility for DNSAddr resolution from a go-ipfs command here? Otherwise, this makes sense since we already have ipfs name resolve and ipfs resolve

[lidel]

There is no utility for resolving /dnsaddr, and DNSLink works better with ipfs resolve -r anyway.

I think it is safe for this to be be deprecated like we did with tar commands in https://github.com/ipfs/go-ipfs/pull/8849/files