CHANGES.md: update for 3.0.19

CHANGES.md: update for 3.0.19

3.0.19 CHANGES.md includes the following:
 * https://github.com/openssl/openssl/pull/29214
   "s390x: Check and fail on invalid malformed ECDSA signatures"
 * https://github.com/openssl/openssl/pull/29256
   "Clang format 3.0"

Co-Authored-by: Tomáš Mráz <tomas@openssl.org>
Signed-off-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
MergeDate: Mon Jan 26 20:05:45 2026