socket-bind: use lowercase "ipv4"/"ipv6" spelling

socket-bind: use lowercase "ipv4"/"ipv6" spelling

In most of our codebase when we referenced "ipv4" and "ipv6" on the
right-hand-side of an assignment, we lowercases it (on the
left-hand-side we used CamelCase, and thus "IPv4" and "IPv6"). In
particular all across the networkd codebase the various "per-protocol
booleans" use the lower-case spelling. Hence, let's use lower-case for
SocketBindAllow=/SocketBindDeny= too, just make sure things feel like
they belong together better.

(This work is not included in any released version, hence let's fix this
now, before any fixes in this area would be API breakage)

Follow-up for #17655