Remove server allowlist

[distantnative]

This PR …

• Do we want to display the full server name in the Panel now or do we still want to clean it up?

Enhancement

• Remove hard-coded restriction of supported servers https://feedback.getkirby.com/567

Docs

• Remove hard server requirements in favor of something like

  Apache works out of the box, nginx and Caddy work with our cookbook recipes and other servers can be used by advanced users

• Remove servers option

Ready?

• In-code documentation (wherever needed)
• Unit tests for fixed bug/feature
• Tests and checks all pass

For review team

• Add changes & docs to release notes draft in Notion

[lukasbestle]

Do we want to display the full server name in the Panel now or do we still want to clean it up?

Since SERVER_SOFTWARE is an environment variable, it can be set to whatever value. I briefly looked for standards, but it seems like there is none. So we cannot expect the value to be set in a uniform way. For debugging it may actually be useful to have access to the full string including version, so I think we could keep it like this.