advice: suggest using subcommand "git config set"

advice: suggest using subcommand "git config set"

The advice message currently suggests using "git config advice..." to
disable advice messages, but since

00bbdde141 (builtin/config: introduce "set" subcommand, 2024-05-06)

we have the "set" subcommand for config. Since using the subcommand is
more in-line with the modern interface, any advice should be promoting
its usage. Change the disable advice message to use the subcommand
instead. Change all uses of "git config advice" in the tests to use the
subcommand.

Signed-off-by: Bence Ferdinandy <bence@ferdinandy.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>