net/url: allow IP-literals with IPv4-mapped IPv6 addresses
The security fix we applied in CL709857 was overly broad. It applied
rules from RFC 2732, which disallowed IPv4-mapped IPv6 addresses, but
these were later allowed in RFC 3986, which is the canonical URI syntax
RFC.
Revert the portion of CL709857 which restricted IPv4-mapped addresses,
and update the related tests.
Fixes #75815
Change-Id: I3192f2275ad5c386f5c15006a6716bdb5282919d
Reviewed-on: https://go-review.googlesource.com/c/go/+/710375
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Ethan Lee <ethanalee@google.com>
Auto-Submit: Roland Shoemaker <roland@golang.org>