Add Ipv6Addr::to_ipv4_mapped

[nanpuyue]

• add Ipv6Addr::to_ipv4_mapped
• deprecate Ipv4Addr::to_ipv6_compatible & Ipv6Addr::to_ipv4 reference: Add a note for Ipv4Addr::to_ipv6_compatible #75150

According to IETF RFC 4291, the "IPv4-Compatible IPv6 address" is deprecated.

2.5.5.1. IPv4-Compatible IPv6 Address

The "IPv4-Compatible IPv6 address" was defined to assist in the IPv6
transition. The format of the "IPv4-Compatible IPv6 address" is as
follows:

| 80 bits | 16 | 32 bits |
+--------------------------------------+--------------------------+
|0000..............................0000|0000| IPv4 address |
+--------------------------------------+----+---------------------+

Note: The IPv4 address used in the "IPv4-Compatible IPv6 address"
must be a globally-unique IPv4 unicast address.

The "IPv4-Compatible IPv6 address" is now deprecated because the
current IPv6 transition mechanisms no longer use these addresses.
New or updated implementations are not required to support this
address type.

And the current implementation of Ipv4Addr::to_ipv6_compatibleis incorrect: it does not check whether the IPv4 address is a globally-unique IPv4 unicast address.

Please let me know if there are any issues with this pull request.