Only support kebab case labels in Spin.toml

[kate-goldenring]

Now that 3.0 is approaching, this may be a good time to be more defined and consistent about the syntax of labels in spin.toml. Right now, we support both kebab-case and snake_case. We should consider dropping support for snake_case.

See this issue for more background: #2278

[lann]

I think we would probably want to do this in a spin manifest revision (i.e. manifest v3) rather than Spin 3.0 per se.