Don't use x509-validation fork

[infinisil]

We're currently using the x509-validation fork from https://github.com/vincenthz/hs-certificate/pull/126 in order to have a pure function to do X.509 verification. However we need to upload this library to Hackage, which can't work with this. So we need to do one of these things, ordered from most to least preferred:

• Get the PR merged and a new x509-validation release published by @vincenthz
• Copy the pure validation code from x509-validation library into our own source code and use that instead
• Make this library have its main functions be in IO

https://github.com/tweag/haskell-fido2/blob/44e89a1519b7d9a809209a3e4907daf782091650/cabal.project#L3-L10

[ErinvanderVeen]

The PR has just been merged, but we do need to do version bumb and ask for a re-upload to hackage.