various: Master is now main

Also cleans out various old strings from the translation strings.

cmd/stcrashreceiver/sentry.go

@@ -111,7 +111,7 @@ func parseReport(path string, report []byte) (*raven.Packet, error) {
 		loader.LockWithVersion(version.tag)
 	} else {
 		// Last resort
-		loader.LockWithVersion("master")
+		loader.LockWithVersion("main")
 	}
 	defer loader.Unlock()
 

cmd/strelaysrv/README.md

@@ -121,11 +121,11 @@ Relay related libraries used by this repo
 ----
 ##### Relay protocol definition.
 
-[Available here](https://github.com/syncthing/syncthing/tree/master/lib/relay/protocol)
+[Available here](https://github.com/syncthing/syncthing/tree/main/lib/relay/protocol)
 
 
 ##### Relay client
 
 Only used by the testutil.
 
-[Available here](https://github.com/syncthing/syncthing/tree/master/lib/relay/client)
+[Available here](https://github.com/syncthing/syncthing/tree/main/lib/relay/client)