ipv6 support for remote addresses added

[eedugon]

This PR describes that the Stack 9.2 supports IPv6 addresses when configuring a remote cluster within Kibana.

I don't want to highlight that much in our docs, as it doesn't feel necessary (we didn't even mention that we supported IPv4 at all, neither that IPv6 wasn't supported by Kibana (it was supported by Elasticsearch)).

Extra improvements:

• The ECE -> self-managed and ECH -> self-managed docs have been improved and converted to snippets.
• Also the Kibana specific steps have been improved to match the reality.

Not updated (out of the scope of this PR)

• The steps for the self-managed docs haven't been improved because we will be rewriting them soon in a different PR.
• For all other remote clusters use cases, the Kibana UI fixes will come in another PR. This one contains only the changes related with the IPv6 support. I already have a second snippet ready to be used by 6 other docs.

Closes #3093

[github-actions]

🔍 Preview links for changed docs

• deploy-manage/remote-clusters/ec-remote-cluster-self-managed.md
• deploy-manage/remote-clusters/ece-remote-cluster-self-managed.md
• deploy-manage/remote-clusters/remote-clusters-api-key.md
• deploy-manage/remote-clusters/remote-clusters-cert.md

[eedugon]

Added extra fixes, as the "Elasticsearch API method" of both ECE -> self-managed and ECH -> self-managed docs were incorrect!
I've merged both to a snippet and fixed the incorrect statements.

[shainaraskas]

couple of comments for you. I think the applies tag needs to be removed from the api instructions

[florent-leborgne]

I think this works. Few minor non blocking suggestions.
Thanks Edu for handling the Kibana part :)

[shainaraskas]

agree with florent's comments. otherwise good to go

[eedugon]

I'm sorry I've been fully occupied the last few days by another PR :( I'll try to get to this tomorrow.
@shainaraskas if you want to take over feel free to go ahead and commit + merge whatever you consider :)