Upsun/platform name changes

[rfay]

Is there an existing issue for this?

• I have searched the existing issues

Is your feature request related to a problem?

Platform.sh has changed its name and branding

Describe your solution

• Update ddev-webserver build as needed (I'm not sure if they have changed their apt repositories)
• Update docs to include new names (but leave the old in there for search)
• the platformsh cli retains the name platform, so nothing to be done there

Describe alternatives

No response

Additional context

No response