prepare for 0.26.0 release

[kschoche]

## 0.26.0 (October 26, 2023)

Changes:
* Default `vault` version updated to 1.15.1
* Default `vault-k8s` version updated to 1.3.1
* Tested with Kubernetes versions 1.24-1.28
* server: OpenShift default readiness probe returns 204 when uninitialized [GH-966](https://github.com/hashicorp/vault-helm/pull/966)

Features:
* server: Add support for dual stack clusters [GH-833](https://github.com/hashicorp/vault-helm/pull/833)
* server: Support `hostAliases` for the StatefulSet pods [GH-955](https://github.com/hashicorp/vault-helm/pull/955)
* server: Add `server.service.active.annotations` and `server.service.standby.annotations` [GH-896](https://github.com/hashicorp/vault-helm/pull/896)
* server: Add long-lived service account token option [GH-923](https://github.com/hashicorp/vault-helm/pull/923)

Bugs:
* csi: Add namespace field to `csi-role` and `csi-rolebindings`. [GH-909](https://github.com/hashicorp/vault-helm/pull/909)

Improvements:
* global: Add `global.namespace` to override the helm installation namespace. [GH-909](https://github.com/hashicorp/vault-helm/pull/909)
* server: use vault.fullname in Helm test [GH-912](https://github.com/hashicorp/vault-helm/pull/912)
* server: Allow scaling HA replicas to zero [GH-943](https://github.com/hashicorp/vault-helm/pull/943)

[thyton]

LGTM!