mbedtls: update to 3.6.2

mbedtls: update to 3.6.2

Fixes the following security problem:
* CVE-2024-49195: Fix a buffer underrun in mbedtls_pk_write_key_der()
  when called on an opaque key, MBEDTLS_USE_PSA_CRYPTO is enabled, and
  the output buffer is smaller than the actual output. Fix a related
  buffer underrun in mbedtls_pk_write_key_pem() when called on an opaque
  RSA key, MBEDTLS_USE_PSA_CRYPTO is enabled and MBEDTLS_MPI_MAX_SIZE is
  smaller than needed for a 4096-bit RSA key.

Link: https://github.com/openwrt/openwrt/pull/16768
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>