Update CHANGELOG.md - modifications to Contract.TxConstraints.mustPayTo*

[marcusbfs]

This includes the modifications to Contract.TxConstraints.mustPayTo* functions.

Pre-review checklist

• All code has been formatted using our config (make format)
• Any new API features or modification of existing behavior are covered with tests
• The template (templates/ctl-scaffold) has been updated
• The changelog has been updated under the ## Unreleased header, using the appropriate sub-headings (### Added, ### Changed, ### Removed, ### Fixed), and the links to the appropriate issues/PRs have been included