api+cmd+mod: update to btcd v0.26 and v2 modules

[guggero]

Depends on benma/miniscript-go#1.

Maintenance update that bumps all btcd related dependencies to the newest version, which includes a switch to the untangled v2 modules.
This is the second PR in a 3-part series that's required to eventually get the same dependency updates into bitbox-wallet-app.

Opening as draft until the dependent PR is merged and go.mod + go.sum are updated.

[guggero]

The linter failure is because of the temporary replace rule, which will be removed once the PR is out of draft:

[6](https://github.com/BitBoxSwiss/bitbox02-api-go/actions/runs/32944343574/job/98101749457?pr=179#step:4:39)
  go.mod:38:1: replacement are not allowed: github.com/benma/miniscript-go (gomoddirectives)
  replace github.com/benma/miniscript-go => github.com/thalia-finance/miniscript-go v0.0.0-20260721123813-d82de38591a3

[benma]

miniscript-go is merged, please rebase 🙏

[guggero]

Thanks! Updated.