Update the pyca-cryptography submodule to version 1.8.1

It was released a couple of days after our latest update

[extended tests]
Reviewed-by: NTim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3346)

pyca-cryptography @ 6c5a519f

-Subproject commit 722235c46721acfe8b601e7846730c3c1fa588c5
+Subproject commit 6c5a519f9b382a1746c1bda5f01aaeb2809250ef