bump secp v0.7.1->v0.8.0

[SomberNight]

note:

$ cd libsecp256k1
$ git verify-commit 6e2c8bc4ecdc6e71dbe7a368f360d8d453ce435d
gpg: Signature made Mon Aug  3 21:51:27 2026 GMT
gpg:                using RSA key 6A8F9C266528E25AEB1D7731C2371D91CB716EA7
gpg: Good signature from "Sebastian Falbesoner (theStack) <sebastian.falbesoner@gmail.com>" [unknown]
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: 6A8F 9C26 6528 E25A EB1D  7731 C237 1D91 CB71 6EA7