Add required_conan_version to all recipes which don't have it yet

[Croydon]

Since the required_conan_version attribute got added in Conan 1.28.0 this should also be the default value as everything else would be without effect anyway

required_conan_version = ">=1.28.0"

Please note that this does NOT automatically mean that all older Conan versions are 100% unsupported. But we have a tendency to live on the edge anyway. And again: Older Conan clients will ignore the attribute and there is effectively no difference to the situation right now.

Adding this attribute make things more transparent and clear and if a client is too old there is a clear message instead of a confusing internal error message.

Having this attribute in every recipe will also encourage contributors to maintain this attribute's information.

I will add it to the templates and update Conventions to add it to all existing recipes