Update Baseline status for custom elements

[foolip]

mdn/content#31764 was filed about the wrong Baseline status shown on https://developer.mozilla.org/en-US/docs/Web/API/CustomElementRegistry/define

At least part of the problem is the builtin custom elements is included in the feature definition:

https://github.com/web-platform-dx/web-features/blob/2eed99ed8ff87533fe34c017fd192932f8c97e0a/feature-group-definitions/custom-elements.yml#L12C31-L12C54

We need either an editorial override, or to split builtin custom elements into a separate feature.

[foolip]

We also need to think about what to do with :host-context() not being supported in Firefox.

[foolip]

This was made obsolete by #1089.