ppp: update Kconfig help message

ppp: update Kconfig help message

Both links of the PPPoE section are no longer valid, and the CVS version
is no longer relevant.

- Replace the TLDP URL with the pppd project homepage.
- Update pppd version requirement for PPPoE.
- Update RP-PPPoE project homepage, and clarify that it's only needed
  for server mode.

Signed-off-by: Qingfang Deng <qingfang.deng@linux.dev>
Reviewed-by: Julian Braha <julianbraha@gmail.com>
Link: https://patch.msgid.link/20260402050053.144250-1-qingfang.deng@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>