uapi: in6: replace temporary label with rfc9486

Not really a fix per se, but IPV6_TLV_IOAM is still tagged as "TEMPORARY
IANA allocation for IOAM", while RFC 9486 is available for some time
now. Just update the reference.

Fixes: 9ee11f0f ("ipv6: ioam: Data plane support for Pre-allocated Trace")
Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240226124921.9097-1-justin.iurman@uliege.be


Signed-off-by: Jakub Kicinski <kuba@kernel.org>