Upgrade to secp256k1 0.6.0 and MuSig2

[msgilligan]

secp256k1 0.6.0 has been released and adds support for MuSig2.

Since our build procedure currently (by default) uses Nixpkgs -- NixOS/nixpkgs#354528 has been merged -- secp256k1 0.6.0 is now available via Nix.

TODO list:

• update README.adoc -- PR README: Update secp256k1 installation to v0.6.0 #131
• update flake.lock -- PR flake.lock: nix flake update #118 (upgraded to secp256k1 0.6.0)
• re-run jextract and generate updated wrapper classes. -- PR Upgrade jextract'ed headers to latest JDK 23 jextract #121
• add the new secp256k1_ec_pubkey_sort function to our API. (from 0.5.0)
• add MuSig2 and other 0.6.0 API additions to secp-jdk API.