Resolve "Release Nextcloud chart version 0.15.13"

Thank you for making a merge request. Before you merge this, please make sure that you:

• Update CHANGELOG.md.

Test in stackspin/stackspin (if applicable)

If you made very big changes to the chart, you'll want to test whether the chart still works before releasing the new version. To do so, create an MR in the stackspin/stackspin repository, change flux2/core/base/sources/nextcloud-helmrepository.yaml to track the unstable helm repository, instead of stable, and change flux2/apps/nextcloud/release.yaml to this MRs chart version.

• Created an MR in stackspin/stackspin that applies the changes from this MR
• Tests in that MR in stackspin/stackspin succeed

Closes #1097 (closed)