Meta Boxes: Prevent the meta box form from being submitted when pressing Enter

[pento]

The meta box form is intended to be submitted when the post is saved, it shouldn't be submitted by pressing Enter whilst inside the form.

[pento]

In 43880:

Block Editor: Prevent the meta box form from being accidentally submitted.

Meta boxes are loaded inside a wrapper form, that the block editor submits as part of the saving process. It shouldn't be submitted manually.

See #45284.

[SergeyBiryukov]

In 44240:

Block Editor: Prevent the meta box form from being accidentally submitted.

Meta boxes are loaded inside a wrapper form, that the block editor submits as part of the saving process. It shouldn't be submitted manually.

Props pento.
Merges [43880] to trunk.
Fixes #45284.