chore(config): resolve primary STUN servers via SRV record (fixes #10029) (#10031)

### Purpose

Fixes #10029

### Testing

```
[3JPXJ] 2025/04/03 14:36:44.601454 stun.go:146: DEBUG: Running stun for Stun@udp://[::]:22000 via fyc5mja4mz5s0vmz1txx.syncthing.net:9999
[3JPXJ] 2025/04/03 14:36:54.185157 stun.go:170: DEBUG: Stun@udp://[::]:22000 stun discovery on fyc5mja4mz5s0vmz1txx.syncthing.net:9999 resulted in no address
[3JPXJ] 2025/04/03 14:36:54.185204 stun.go:146: DEBUG: Running stun for Stun@udp://[::]:22000 via stun.internetcalls.com:3478
```

### Documentation

https://github.com/syncthing/docs/pull/904

lib/config/config.go

@@ -68,13 +68,9 @@ var (
 	DefaultTheme = "default"
 	// Default stun servers should be substituted when the configuration
 	// contains <stunServer>default</stunServer>.
-
-	// DefaultPrimaryStunServers are servers provided by us (to avoid causing the public servers burden)
-	DefaultPrimaryStunServers = []string{
-		// Discontinued because of misuse. See https://forum.syncthing.net/t/stun-server-misuse/23319
-		//"stun.syncthing.net:3478",
-	}
-	DefaultSecondaryStunServers = []string{
+	// The primary stun servers are provided by us and are resolved via an SRV record
+	// The fallback stun servers are used if the primary ones can't be resolved or are down.
+	DefaultFallbackStunServers = []string{
 		"stun.counterpath.com:3478",
 		"stun.counterpath.net:3478",
 		"stun.ekiga.net:3478",