remove obsolete functions

[ndeloof]

see #742

[glours]

We'll need to do a major version as some downstream projects may still use the Load function, no? WDYT @ndeloof ?

[ndeloof]

Load has been deprecated since 1.18 (ad9be94), so they have been warned!

[alexellis]

Thanks for looking at this.

Isn't there a way to mark a method as deprecated in Go through godoc or similar?

https://github.com/compose-spec/compose-go/pull/743/files#diff-8e9af1e483632eac17610237caaa5f879ea7bfb3e1e1ea228e45a17c770b9432L260

Sometimes a struct field, function, type, or even a whole package becomes redundant or unnecessary, but must be kept for compatibility with existing programs. To signal that an identifier should not be used, add a paragraph to its doc comment that begins with “Deprecated:” followed by some information about the deprecation.

I didn't see that on the ParseYAML function.

[ndeloof]

This one was not marker as deprecated as it's just a legacy (not used by the canonical loading sequence as implemented in docker/compose) we forgot to tag as deprecated/remove