[cmake] Update to mbedtls 3.6.7

[tobil4sk]

This solves mac static build errors:

/Users/runner/work/neko/neko/libs/src/MbedTLS/library/ssl_tls13_keys.c:44:5: error: initializer-string for character array is too long, array size is 8 but initializer has size 9 (including the null terminating character); did you mean to use the 'nonstring' attribute? [-Werror,-Wunterminated-string-initialization]
   44 |     MBEDTLS_SSL_TLS1_3_LABEL_LIST
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/runner/work/neko/neko/libs/src/MbedTLS/library/ssl_tls13_keys.h:14:40: note: expanded from macro 'MBEDTLS_SSL_TLS1_3_LABEL_LIST'
   14 |     MBEDTLS_SSL_TLS1_3_LABEL(finished, "finished") \
      |                                        ^~~~~~~~~~

Upstream issue: Mbed-TLS/mbedtls#9944

[Simn]

Thanks, I was about to bring up these CI failures. Feel free to merge this once it's green!