chore: Ship syncthing v2 RC.20

There's code here that should auto-upgrade this when it's outdated, but
it doesn't work for pre-releases. SyncTrayzor would normally not ship
with pre-releases, but this beta phase is an exception.

Author: GermanCoding

Taskfile.yml

@@ -3,7 +3,7 @@
 version: "3"
 
 vars:
-   DOWNLOAD_SYNCTHING_VERSION: "v2.0.0-rc.19"
+   DOWNLOAD_SYNCTHING_VERSION: "v2.0.0-rc.20"
 
 tasks:
    download-syncthing: