Target go 1.27 and let litewitness use crypto/mldsa

Before getting out a new release of torchwood, we think getting onto go 1.27 and using the stdlib mldsa would be a good idea. Should not become an obstacle for debian pkging either (possibly the opposite).

Locate the litewitness entry point and the module or toolchain configuration, then inspect how ML-DSA is currently provided and how Debian packaging is defined. Done means the project targets Go 1.27, litewitness uses the standard-library mldsa package, and Debian packaging remains unobstructed.

Written by the indexing model from the issue text.