string.capitalize different output in C# and GDScript

[AndreSacilotto]

Godot version

3.4.4

System information

Windows 10

Issue description

The C# version of string.Capitilize() has incorrect output. Which is different from the documentation and GDScript.

Problem: "capitalize camelCase mixed_with_underscores".Capitalize();
Expected: Capitalize Camel Case Mixed With Underscores
Received: Capitalize Camelcase Mixed With Underscores

Maybe can have something to do with #28748

Steps to reproduce

Write this in a _Ready function:

"capitalize camelCase mixed_with_underscores".Capitalize();

Minimal reproduction project

Not necessary?

[raulsntos]

Duplicate of #56941.

Fixed by #57076 but only in master, not in 3.x, probably because it's a breaking compatibility change.

[Calinou]

Fixed by #57076 but only in master, not in 3.x, probably because it's a breaking compatibility change.

The different behavior is probably worth documenting in 3.x. Also, could the new behavior be made opt-in in 3.x (e.g. by passing an optional new_capitalize=true argument to .Capitalize() in C#)?

[akien-mga]

I think this can be cherrypicked.

Duplicate of #56941