fix: replace --delete-delay by --delete-before to work on macos

[timdreier]

Topic and Scope

This PR fixes #2441. It replaces the --delete-delay of rsync by --delete-before, because --delete-delay isn't implemented in rsync on macOS.

Concerns/issues

The flag --delete-before deletes removed files before copying the changed files. In this scenario the change of this flag shouldn't make any difference.

Formal requirements

There are some formal requirements that should be satisfied. Please mark those by checking the corresponding box.

• [x] I did check that the app can still be opened and does not throw any browser logs
• [x] I created tests for newly added PHP code (check this if no PHP changes were made)
• [x] I updated the OpenAPI specs and added an entry to the API changelog (check if API was not modified)
• [x] I notified the matrix channel if I introduced an API change