Upgrading webmention to 0.6.0, removes native-tls

[dessalines]

This removed the native-tls dependency, see https://github.com/timmarinin/webmention/pull/6

[Nutomic]

Then you should also be able to remove openssl from docker build files and from documentation.

[dessalines]

We have a custom docker image built for lemmy, that includes some other things (like postgres libraries) also. I'll check the documentation tho.