Bump libsecp256k1 from v0.4.1 to v0.5.0

[kristapsk]

This release includes algorithmic improvements which increase the speed of key generation and signing. At the same time, the binary size has been reduced significantly.

Full changelog: https://github.com/bitcoin-core/secp256k1/blob/master/CHANGELOG.md#050---2024-05-06

The ABI is backward compatible with versions 0.4.x and 0.3.x, so no python-bitcointx update is needed.