Add Unreleased section back to CHANGELOG.md (#1601)

This change fixes the `release` workflow. It has been broken since the
v7.5.0 release, when the `CHANGELOG` was updated by hand instead of with
the `bump_version.sh` script. That is, the `Unreleased` section was
removed from the `CHANGELOG`, which caused the `Create release artifacts`
step of the `release` workflow to fail because it looks for the
`Unreleased` header and does not find it.

Author: maxrake

CHANGELOG.md

@@ -6,6 +6,8 @@ The sections should follow the order `Packaging`, `Added`, `Changed`, `Fixed` an
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
+## Unreleased
+
 ## 7.5.0 - 2025-04-01
 
 ### Changed