helm-docs: add migration guides around PodSecurityPolicy and chart 5.0

docs/sources/helm-charts/mimir-distributed/migration-guides/migrate-helm-chart-4.x-to-5.0.md

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+---
+description: "Migrate the Helm chart from version 4.x to 5.0"
+title: "Migrate the Helm chart from version 4.x to 5.0"
+menuTitle: "Migrate Helm chart 4.x to 5.0"
+weight: 110
+---
+
+# Migrate the Helm chart from version 4.x to 5.0
+
+Grafan Mimir Helm chart version 5.0 and later does not install [PodSecurityPolicy](https://kubernetes.io/docs/concepts/security/pod-security-policy/) objects on Kubernetes version 1.24 by default to prepare for upgrade to Kubernetes version 1.25.
+
+## Prerequisite
+
+- This procedure is only applicable if `rbac.create` is `true` and `rbac.type` is `psp` in your current Helm values. This was the case by default before Helm chart version 5.0.
+
+## Procedure
+
+1. If `rbac.create` is `false` or `rbac.type` is `scc`, then there is nothing to do, skip the whole procedure.
+1. Choose between the following options:
+
+   1. If you are on Kubernetes version 1.24 and want to keep using PodSecurityPolicy, then merge the following setting into your custom values file:
+
+      ```yaml
+      rbac:
+        forcePSPOnKubernetes124: true
+      ```
+
+      > **Note**: Warning: this value prevents you from upgrading to Kubernetes version 1.25! Follow the other option before upgrading to Kubernetes 1.25.
+
+   1. If you have Kubernetes version 1.22 or later and want to upgrade to Kubernetes version 1.25 after this procedure then follow the [Migrate to Kubernetes version 1.25]({{< relref "./migrate-to-kubernetes-version-1.25.md" >}}) guide.

docs/sources/helm-charts/mimir-distributed/migration-guides/migrate-to-kubernetes-version-1.25.md

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+---
+title: "Migrate to Kubernetes version 1.25"
+menuTitle: "Migrate to Kubernetes 1.25"
+description: "Learn how to migrate the Helm chart to Kubernetes version 1.25"
+weight: 10
+---
+
+# Migrate to Kubernetes version 1.25
+
+This procedure describes how to prepare a Grafana Mimir Helm chart deployment for an upgrade to Kubernetes 1.25.
+
+## Background
+
+Kubernetes version 1.25 removes the support for the deprecated [PodSecurityPolicy](https://kubernetes.io/docs/concepts/security/pod-security-policy/) object. You can learn more about this topic by visiting [PodSecurityPolicy Deprecation: Past, Present, and Future](https://kubernetes.io/blog/2021/04/06/podsecuritypolicy-deprecation-past-present-and-future/).
+
+Due to how Helm [works](https://helm.sh/docs/topics/kubernetes_apis/), PodSecurityPolicy objects must already be removed from the deployment on Kubernetes version 1.24. If you are using PodSecurityPolicy on Kubernetes 1.24, this is a breakind change. Grafana Mimir helm chart versions before 5.0 used PodSecurityPolicy by default.
+
+## Prerequisite
+
+- You have Kubernetes version 1.22, 1.23 or 1.24.
+- This procedure is only applicable if `rbac.create` is `true` and `rbac.type` is `psp` in your current Helm values. This was the case by default before Helm chart version 5.0.
+
+## Procedure
+
+1. If `rbac.create` is `false` or `rbac.type` is `scc`, then there is nothing to do, skip the whole procedure.
+1. Optionally follow the Kubernetes [Migrate from PodSecurityPolicy to the Built-In PodSecurity Admission Controller](https://kubernetes.io/docs/tasks/configure-pod-container/migrate-from-psp/) guide to replace PodSecurityPolicy.
+   > **Note**: Grafana Mimir does not require any special permissions on the hosts that it
+   > runs on. Because of this, you can deploy it in environments that enforce the
+   > Kubernetes [Restricted security policy](https://kubernetes.io/docs/concepts/security/pod-security-standards/).
+1. Set the `rbac.create` value to `false`.
+1. Upgrade the deployment. The chart will not install PodSecurityPolicy objects anymore.