ppp: update to 2.5.3

ppp: update to 2.5.3

Update to 2.5.3 and use the official tarball.

Changelog: https://github.com/ppp-project/ppp/releases/tag/v2.5.3

Removed upstreamed:
- 000-pppd-session-fixed-building-with-gcc-15.patch
- 001-pppdump-fixed-building-with-gcc-15.patch
- 501-fix-memcpy-fortify.patch
- 502-remove_mru.patch

Manually rebased:
- 500-add-pptp-plugin.patch

Add a pending patch to relax the check to avoid breaking existing
configurations.

Signed-off-by: Qingfang Deng <dqfext@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23540
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>