Retire `stun.syncthing.net` A record

### What happened?

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1144898

It's not affecting recent Syncthing versions, but we also don't need to keep it around either.

### Syncthing version

any

### Platform & operating system

any

### Browser version

_No response_

### Relevant log output

```shell

```