Stop adding GitHub SSH keys (#171)

We need to fix the SSH keys shipped with this action:
https://github.blog/2023-03-23-we-updated-our-rsa-ssh-host-key/

But, we have another issue
(https://github.com/webfactory/ssh-agent/pull/108) with regards to host
keys: On self-hosted runners which are not ephemeral the known_host file
fills up with repeated entries, because every action run adds a new line
with the same host keys.

Also, on those machines, the old key will still be in the `known_hosts`
file.

IMHO this action should not be repsonsible for shipping SSH host keys,
that's too much responsibility.

This section in the code is a leftover from early days when GitHub
provided runners did not include SSH keys at all. For a long time
already, GH takes care of placing their SSH keys in their runner images.

For self-hosted runners, those people setting up the runner should fetch
and verify SSH keys themselves and put it into the `known_hosts` file.

I know this is a breaking change and is going to annoy users. But on the
other hand, there is no better opportunity to drop this feature than
with an emergency-style key revocation as today.

Closes #106, closes #129, closes #169, closes #170, closes #172.

README.md

@@ -272,4 +272,4 @@ developer looking for new challenges, we'd like to hear from you!
 - <https://www.webfactory.de>
 - <https://twitter.com/webfactory>
 
-Copyright 2019 – 2022 webfactory GmbH, Bonn. Code released under [the MIT license](LICENSE).
+Copyright 2019 – 2023 webfactory GmbH, Bonn. Code released under [the MIT license](LICENSE).