Breaking change: remove default legend and label sizes or else change to l

[edwardhorsford]

The library currently defaults legends and labels to xl if no size is provided and they are page headings.

This differs from the native components where consumers are expected to explicitly provide the size class. It also defaults to xl which is usually not correct - most services should be defaulting to l (formal guidance on this coming).

We could:

1. Remove the default as part of a breaking change, possibly showing a warning for a version or two that teams need to explicitly set the size.

2. Change the default to l as part of a breaking change - again possibly showing a warning?

3. aligns closest with how the native components work, though for teams that haven't set a size, they'll need to set it on each usage of radios / checkboxes / inputs, etc.

4. might end up changing consumer services' pages - though often the change will be to a more appropriate heading size. It may mean fewer changes to templates as in most cases services should be using size l anyway.