clients/horizonclient,txnbuild: update changelogs for v4.1.0

[leighmcculloch]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

Update the changelogs for clients/horizonclient and txnbuild for the combined v4.1.0 release. Also add a missing empty release details to clients/horizonclient changelog that was missing in the previous release which only changed txnbuild.

Why

There are several changes to txnbuild on master that could be released. I specifically want to get the SEP-10 v2.1.0 changes in a release.

Known limitations

There are only meaningful changes to txnbuild but txnbuild and horizonclient get released together, so that is why there are empty changelog release notes for the latter.

[JakeUrban]

I'd like to understand why we opted to put all Go code in one repo, since this is a familiar issue.

[leighmcculloch]

Released in https://github.com/stellar/go/releases/tag/horizonclient-v4.1.0.