Prepare release metadata for openstack.cloud 2.6.0

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>

changelogs/fragments/add_shard_key_field_for_baremetal_node.yaml

@@ -1,3 +0,0 @@
----
-minor_changes:
-  - Add support for setting the shard key on a baremetal node.

changelogs/fragments/baremetal_port_group_module.yaml

@@ -1,5 +0,0 @@
----
-minor_changes:
-  - Added the new ``openstack.cloud.baremetal_port_group`` module to manage
-    Bare Metal port groups (create, update, and delete), including CI role
-    coverage and unit tests.

changelogs/fragments/volume_image_metadata_module.yaml

@@ -1,6 +0,0 @@
----
-minor_changes:
-  - Added a new ``openstack.cloud.volume_image_metadata`` module to manage
-    Cinder volume image metadata via the ``os-set_image_metadata`` API.
-    This enables correct preservation of image provenance and boot semantics
-    for volumes, which cannot be achieved using regular volume metadata.

changelogs/fragments/volume_retype_module.yaml

@@ -1,4 +0,0 @@
----
-minor_changes:
-  - Added a new ``openstack.cloud.volume_retype`` module to retype
-    Cinder volumes via the ``os-retype`` API.