mbedtls: update to 3.6.6

mbedtls: update to 3.6.6

This version fixes some security problems:
 * Client impersonation while resuming a TLS 1.3 session
   (CVE-2026-34873)
 * Entropy on Linux can fall back to /dev/urandom (CVE-2026-34871)
 * PSA random generator cloning (CVE-2026-25835)
 * Compiler-induced constant-time violations (CVE-2025-66442)
 * Null pointer dereference when setting a distinguished name
   (CVE-2026-34874)
 * Buffer overflow in FFDH public key export (CVE-2026-34875)
 * FFDH: lack of contributory behaviour due to improper input validation
   (CVE-2026-34872)
 * Signature Algorithm Injection (CVE-2026-25834)
 * CCM multipart finish tag-length validation bypass (CVE-2026-34876)
 * Risk of insufficient protection of serialized session or context data
   leading to potential memory safety issues (CVE-2026-34877)
 * Buffer underflow in x509_inet_pton_ipv6() (CVE-2026-25833)

Changelog: https://github.com/Mbed-TLS/mbedtls/releases/tag/mbedtls-3.6.6

Size increases by 470 bytes on aarch64:
343995 bin/packages/aarch64_generic/base/libmbedtls21-3.6.5-r1.apk
344465 bin/packages/aarch64_generic/base/libmbedtls21-3.6.6-r1.apk

Link: https://github.com/openwrt/openwrt/pull/22787
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>