Rename "Form label" to "Form field label"

[Robbert]

The NL Design System community has approved the name "Form field label", we it would be best to rename our component too.

https://nldesignsystem.nl/form-field-label

This is breaking change, with significant impact on the community. Many projects use the utrecht-form-label class name, and many themes use the utrecht.form-label design tokens.

We should investigate how:

• we can help folks with the migration, for example: warning them when they use outdated class names and design tokens
• how we can connect with the users of this component