Bump galaxy.yml to 2.6.0.
Add a 2.6.0 entry in changelogs/changelog.yaml,
regenerate CHANGELOG.rst with antsibull-changelog.
Tests-Run: tox -e pep8
Change-Id: I03a4a424d1f47a9fdaa70efae9b8c9a3fa60b072
Signed-off-by: Grzegorz Koper <grzegorzk@stackhpc.com>