Prepare Release v2.6.0 (#777)

### Description
Prepare of Release v2.6.0

### Issues Resolved
#776

### Check List
- [x] Commits are signed per the DCO using --signoff 
- [ ] Unittest added for the new/changed functionality and all unit
tests are successful
- [ ] Customer-visible features documented
- [x] No linter warnings (`make lint`)

If CRDs are changed:
- [x] CRD YAMLs updated (`make manifests`) and also copied into the helm
chart
- [x] Changes to CRDs documented

Please refer to the [PR
guidelines](https://github.com/opensearch-project/opensearch-k8s-operator/blob/main/docs/developing.md#submitting-a-pr)
before submitting this pull request.

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and
signing off your commits, please check
[here](https://github.com/opensearch-project/OpenSearch/blob/main/CONTRIBUTING.md#developer-certificate-of-origin).

Signed-off-by: Prudhvi Godithi <[email protected]>

charts/opensearch-cluster/Chart.yaml

@@ -2,5 +2,5 @@ apiVersion: v2
 name: opensearch-cluster
 description: A Helm chart for OpenSearch Cluster
 type: application
-version: 2.5.1
-appVersion: 2.5.1
+version: 2.6.0
+appVersion: 2.6.0

charts/opensearch-operator/Chart.yaml

@@ -15,10 +15,10 @@ type: application
 # This is the chart version. This version number should be incremented each time you make changes
 # to the chart and its templates, including the app version.
 # Versions are expected to follow Semantic Versioning (https://semver.org/)
-version: 2.5.1
+version: 2.6.0
 
 # This is the version number of the application being deployed. This version number should be
 # incremented each time you make changes to the application. Versions are not expected to
 # follow Semantic Versioning. They should reflect the version the application is using.
 # It is recommended to use it with quotes.
-appVersion: 2.5.1
+appVersion: 2.6.0