Update CHANGELOG.md (#26215)

To follow new processes for creating release notes on GitHub, I need to update the changelog on the release branch. I've opted to copy the entirety of the 1.15 changelog content to the release branch, adding the notes for 1.15.7.

Next I'll create the tag per these instructions https://github.com/hashicorp/engineering-docs/blob/main/consul/releases/release-process.md#manually-create-github-releases-for-ent-only-patch-releases, and update the release notes for that tag & release.