Fix subPath on datadir mounts (fixes #531)

[Deaddy]

• subPath now does not necessarily end on /data, in particular empty subPath possible for datadir on specific PVC
• datadir persistence now does not depend on overall persistence anymore
• bump .minor because old installs will need to add /data back to the subPath in the values file

Pull Request

Description of the change

Fix #531

The datadir had the subPath-Suffix "/data" forced upon it and it was impossible

Possible drawbacks

If someone actually used the a different volume in the past, they will now need to add /data to their subPath value.

Applicable issues

• fixes #531

Additional information

Checklist

• [x] DCO has been signed off on the commit.
• [x] Chart version bumped in Chart.yaml according to semver.
• [x] (optional) Variables are documented in the README.md
• [x] tested installing and running fresh nextcloud instance with existing persistent volume for datadir