Standardize spelling: Grey to Gray

[SignorParabellaux]

Some places in the project used Grey while others used Gray.
Since in IT the American spelling Gray is more common, this PR updates everything to match.
Just a small cleanup to keep things consistent and easier to read for everyone.

[zdpcdt]

Semi.Avalonia is implemented based on Semi Design, the spelling of Grey is also follow Semi Design standard.
You could see the Tokens for the reason why using Grey.

[SignorParabellaux]

@zdpcdt Thanks for the reply! I understand that Semi Design uses Grey and it makes sense to stay consistent with the design system.

In .NET and Avalonia the spelling Gray is standard. For example Avalonia.Media.Colors.Gray. In CSS both gray and grey exist, but Avalonia itself only supports Gray.

Maybe it could be good to support both spellings. Grey would stay for compatibility with Semi Design, and Gray would make it more natural for developers who already use Avalonia and .NET.

That way we keep the link with Semi Design and also make things a bit smoother for Avalonia developers.

[rabbitism]

Since this is a very breaking change, we can only target 12.0 branch. But we still need some evaluation before making final decision.