[feature] instanceSelectors should be made immutable

[weisdd]

Describe the bug

As it was highlighted in #1497 (comment), the operator does not clean up resources (across Grafana instances) that no longer match instanceSelector.

Version
v5.9.0

To Reproduce
Steps to reproduce the behavior:

0. Deploy `Grafana`

apiVersion: grafana.integreatly.org/v1beta1
kind: Grafana
metadata:
  name: grafana
  labels:
    dashboards: "grafana"
spec:
  config:
    log:
      mode: "console"
    auth:
      disable_login_form: "false"
    security:
      admin_user: root
      admin_password: secret

1. Deploy `GrafanaDashboard` that matches the `Grafana` instance

apiVersion: grafana.integreatly.org/v1beta1
kind: GrafanaDashboard
metadata:
  name: grafanadashboard-sample
spec:
  resyncPeriod: 30s
  instanceSelector:
    matchLabels:
      dashboards: "grafana"
  json: >
    {
      "id": null,
      "title": "Simple Dashboard",
      "tags": [],
      "style": "dark",
      "timezone": "browser",
      "editable": true,
      "hideControls": false,
      "graphTooltip": 1,
      "panels": [],
      "time": {
        "from": "now-6h",
        "to": "now"
      },
      "timepicker": {
        "time_options": [],
        "refresh_intervals": []
      },
      "templating": {
        "list": []
      },
      "annotations": {
        "list": []
      },
      "refresh": "5s",
      "schemaVersion": 17,
      "version": 0,
      "links": []
    }

2. Adjust `instanceSelector` in `GrafanaDashboard`, so it no longer matches the `Grafana` instance

apiVersion: grafana.integreatly.org/v1beta1
kind: GrafanaDashboard
metadata:
  name: grafanadashboard-sample
spec:
  resyncPeriod: 30s
  instanceSelector:
    matchLabels:
      dashboards: "grafana2"
  json: >
    {
      "id": null,
      "title": "Simple Dashboard",
      "tags": [],
      "style": "dark",
      "timezone": "browser",
      "editable": true,
      "hideControls": false,
      "graphTooltip": 1,
      "panels": [],
      "time": {
        "from": "now-6h",
        "to": "now"
      },
      "timepicker": {
        "time_options": [],
        "refresh_intervals": []
      },
      "templating": {
        "list": []
      },
      "annotations": {
        "list": []
      },
      "refresh": "5s",
      "schemaVersion": 17,
      "version": 0,
      "links": []
    }

Expected behavior

The dashboard should be gone from Grafana.

Runtime (please complete the following information):

• Grafana Operator Version: v5.9.0
• Environment: Kubernetes

[theSuess]

Cleaning up resources on a change of the instanceSelector is difficult to realize since the operator only ever knows about the current version of the resource.

This is also the reason as to why the selector field of deployments and stateful sets are immutable.

To be consistent with the rest of the Kubernetes ecosystem, this should be resolved by setting the instanceSelector to be immutable like this: https://kubernetes.io/blog/2022/09/29/enforce-immutability-using-cel/#immutablility-after-first-modification