We want to

• avoid currentItemProvider usage with composite/multis/switchables, b/c it leads to issues like MGNLUI-5886 and
• simplify yaml definitions further.

Thus we're adding default to JcrMultiFieldDefinition

This does not pose risk to compatibility because formerly you had to specify the itemProvider. This only takes effect when itemProvider is not specified which before would result in exception.

Also, modify the example in https://documentation.magnolia-cms.com/display/DOCS62/Multi+field for JCR Multi Field not to use currentItemProvider there. WIth this change, the whole itemProvider part can (and should) be removed. If we want to how something there still, use jcrChildNodeProvider instead to avoid suggesting CIP - bad practice.