syncthing: update to 0.14.39.

This is a regularly scheduled stable release.

Resolved issues:

    #4357: Removing paused folders no longer triggers a crash.
    #4360: Add further security related HTTP headers
    #4375: Improve info level logging in some cases
    #4377: Improve GUI tooltips in chromium based browsers
    #4382: Hide temporary files on Windows
    #4387: Add -device-id command line switch

In addition, failure to upgrade folder markers from file to directory type is
no longer fatal.

(wiz)