Changelog instructions

[willhickey]

Problem

• The changelog includes instructions for creating a new section for every patch release. This was the original plan but we've never done it
• The channels and edge sections at the top of the changelog aren't useful in the release branches and quickly get out of date.

Summary of Changes

• Remove the patch release instructions that we've never used
• Add instructions for removing the misleading information after creating a new branch. See v2.2 Remove channels section from CHANGELOG.md #6739 and v2.3 Remove channels section from CHANGELOG.md #6738 for examples of how this should be done.