CMK-34961: update rustls-webpki to 0.103.15

The update fixes RUSTSEC-2026-0049, RUSTSEC-2026-0098, RUSTSEC-2026-0099
and RUSTSEC-2026-0104. None of these issues affected our usage of the
crate, which is why vulnerability_info.yml carried not_affected entries
for them. These entries are now stale and are removed.

The ticket asks for 0.103.12, but RUSTSEC-2026-0104 is only fixed in
0.103.13, so we take the newest 0.103 release instead.

Change-Id: Ie1cbb20a00eaa21a2135ee0b6d4de43b6bcf5078