Update secp256k1 submodule

[sstone]

Counter-based nonce generation has been extracted into a separate method.

[t-bast]

I don't think we need to merge any update to the libsecp256k1 submodule until they officially ship musig2. I'd rather wait for the musig2 PR to be merged on their side before we update to a "final" version.

[sstone]

We can wait. The musig2 branch that we use is behind secp2561k1's musig2 PR but there are only minor changes and an API change (counter-based nonce generation). I hope it will be released soon though :)

[t-bast]

Looks mostly good to me, we should make a release now that we depend on an official release of secp256k1! However it looks like you're still pointing the submodule to jonasnick's fork of the repository, can you fix it and point it to https://github.com/bitcoin-core/secp256k1?

[sstone]

Looks mostly good to me, we should make a release now that we depend on an official release of secp256k1! However it looks like you're still pointing the submodule to jonasnick's fork of the repository, can you fix it and point it to https://github.com/bitcoin-core/secp256k1?

Done in 6a68a9d