Update qgis-plugin-ci requirement in /requirements

Updates the requirements on [qgis-plugin-ci](https://github.com/opengisch/qgis-plugin-ci) to permit the latest version.
- [Release notes](https://github.com/opengisch/qgis-plugin-ci/releases)
- [Changelog](https://github.com/opengisch/qgis-plugin-ci/blob/master/CHANGELOG.md)
- [Commits](opengisch/qgis-plugin-ci@2.6.0...2.8.0)

---
updated-dependencies:
- dependency-name: qgis-plugin-ci
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements/packaging.txt

@@ -1,4 +1,4 @@
 # Packaging
 # ---------
 
-qgis-plugin-ci>=2.6,<2.8
+qgis-plugin-ci>=2.6,<2.9