Bad recommendation of cipher suites in examples and docs

The http2 server and client examples in the docs use DHE, see towards the end:
```
ctx.set_ciphers("ECDHE+AESGCM:ECDHE+CHACHA20:DHE+AESGCM:DHE+CHACHA20")
```

I think DHE should be removed from this considering [CVE-2002-20001](https://github.com/advisories/GHSA-jx4r-qc68-xjr5)(https://github.com/advisories/GHSA-jx4r-qc68-xjr5)