Let’s Encrypt certificates for TURNS are rejected

TURNS doesn't work.

With
```turn_uris: [ "turns:domain.tld:5349?transport=udp", "turns:domain.tld:5349?transport=tcp" ]```
or
```turn_uris: [ "turns:domain.tld:3478?transport=udp", "turns:domain.tld:3478?transport=tcp" ]```

turnserver reports an error: ```session closed ... reason: TLS/TCP socket buffer operation error (callback)```

riot-web works fine, ```testssl.sh domain.tld:5349``` and ```testssl.sh domain.tld:3478``` also shows no errors.

Start by reproducing the failure with the supplied TURNS URIs on the iOS app and compare it with the working riot-web and testssl.sh results. Trace the app's TURNS/TLS connection path; done means both UDP and TCP configurations establish a connection without the reported TLS/TCP socket buffer error.

Written by the indexing model from the issue text.