Docs: Update router setup guide, new OpenWrt package manager

https://github.com/ivpn/moddns/blob/71e3fcc32ba7b66e412d71e61e44744b5ccf2d8c/app/src/pages/setup/guides/Routers.tsx#L56

Replace lines 56 and 57 with
```
`# OpenWrt 24.10 and older use opkg package manager\n` +
`opkg update\n` +
`opkg install https-dns-proxy\n\n` +
`# OpenWrt 25.12 and newer use apk package manager\n` +
`apk update\n` +
`apk add https-dns-proxy\n\n` +
```

--

Thanks.
