release steps: automate CHANGELOG version header creation

[emidoots]

Today our release process https://github.com/sourcegraph/sourcegraph/issues/13374 requires we manually edit the CHANGELOG to change "Unreleased" to the current version and add a new section for "Unreleased" that is empty:

• Add a new section ## 3.MINOR to CHANGELOG.md immediately under ## Unreleased changes. Add new empty sections under ## Unreleased changes (example).
• Commit this CHANGELOG edit to main using a PR.
• Create the 3.20 branch off the CHANGELOG commit in the previous step: git branch 3.20 && git push origin 3.20.

These steps should be automated as part of our release scripts.