report: place Upload() on io.systemd.Report.Uploader rather than io.systemd.Report interface

We really want to use io.systemd.Report for the interface
provided by systemd-report itself, not by its backend. hence, rename the
interface that uploading plugins shall implement to
io.systemd.Report.Uploader.

Note that we ideally should have a varlink interface definition for that
interface. if we had, we'd have noticed that earlier.

src/report/report-upload.c

@@ -249,7 +249,7 @@ static int upload_collected(Context *context, sd_json_variant *report) {
 
         ssize_t jobs = varlink_execute_directory(
                         REPORT_UPLOAD_DIR,
-                        "io.systemd.Report.Upload",
+                        "io.systemd.Report.Uploader.Upload",
                         params,
                         /* more= */ false,
                         arg_network_timeout_usec,