Landing: remove projects that don't use Cirrus CI (anymore)

[bartekpacia]

It's sad to see for me but these projects have migrated off of Cirrus CI:

• Ruby: https://github.com/ruby/ruby/pull/8679
• Flutter: https://github.com/flutter/flutter/pull/133575
• curl: https://github.com/curl/curl/pull/14244

I was not able to determine that Cirrus CI was ever used as part of Rust's actual infra. I only found this issue: https://github.com/rust-lang/rust/issues/55461

[edigaryev]

I was not able to determine that Cirrus CI was ever used as part of Rust's actual infra.

It seems to be still used, not for the core repository, though: https://github.com/search?q=org%3Arust-lang+path%3A.cirrus.yml&type=code.

[bartekpacia]

Oh, that's great to see – I missed that. Will update.