syncthing: update to 1.0.0.

Based on PR 53878 by David J. Weller-Fahy

This major release honors the fact that Syncthing is used widely
and since a while no longer "really in beta". It does not represent
a breaking change.

Bugfixes:

    #5324: Incorrect warning when --home and path on same directory level
    #5369: Empty file field in log when normalizePath failed
    #5372: Event system can cause deadlock

Enhancements:

    #2760: Option to limit max simultaneous scans
    #5286: Add hardening options to systemd units
    #5320: Show configured rate limit in the GUI
    #5336: Show list of locally changed files for receive only folders

Other issues:

    #5334: New chmduquesne/rollinghash breaks tests
    #5362: lib/model: TestDeregister* tests are bad

(wiz)