Non-translatable fields validation error when using Layout builder

Problem/Motivation

There is an issue when you use Layout builder and Entity blocks on a multilingual website. If you try to translate already existing page, you will get an error message:

Non-translatable fields can only be changed when updating the original language.

The reason for this is that the Layout builder compares source lang and translation lang values and in order for this comparison to work, it converts the sections to arrays. This is happening in LayoutSectionItemList.php file under the equals() method.

EntityBlock exposes it's label as a FormattableMarkup object, which breaks the comparison. It needs a string for the comparison to work.

Steps to reproduce

1. Go to a multilingual website and enable Layout builder and Entity blocks modules.
2. Enable Landing page on the Article content type.
3. Create Article node and attack Entity block to it.
4. Try to create a translation of that article in another language.
5. Enjoy the EntityUntranslatableFieldsConstraint

Proposed resolution

Prepared a patch that converts the FormattableMarkup to a string and allows the comparison to work without a problem.

User interface changes

Since the labels are saved on submit, a re-submit of all existing Entity blocks is needed.