PB-7884: Add PVC size as helm parameter

[ss-px]

What this PR does / why we need it:

• Add PVC size as configurable helm parameter for pre-upgrade hook, with default 65Gi (64 Gi for Mongo dump + 1 Gi for mySQL dump)
• Add mongo6 image to versions list
• Add note regarding PVC creation
• Update image tag from sb to base

Which issue(s) this PR fixes (optional) Closes #PB-7884

To be cherry-picked to 2.8.0

[root@ss-57-0 ~]# helm upgrade px-central https://github.com/portworx/helm/raw/PB-7884/stable/px-central-2.7.3.tgz -n px-backup -f current.yaml --debug --set persistentStorage.preUpgradeHookVolumeSize="90Gi"
[root@ss-57-0 ~]# k get pvc
NAME                                                                                       STATUS   VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS   VOLUMEATTRIBUTESCLASS   AGE
alertmanager-px-backup-alertmanager-db-alertmanager-px-backup-alertmanager-0               Bound    pvc-220f282e-d884-4adc-81f6-237108f9fdbf   2Gi        RWO            portworx-sc    <unset>                 5d12h
alertmanager-px-backup-alertmanager-db-alertmanager-px-backup-alertmanager-1               Bound    pvc-553e63bd-2408-4ba5-8d5a-5a17d7912c24   2Gi        RWO            portworx-sc    <unset>                 5d12h
pre-upgrade-data-pvc                                                                       Bound    pvc-1e46196a-0d93-4d13-a4f3-706780d647a4   90Gi       RWO            portworx-sc    <unset>                 91s