Update pattern overrides handler to retrieve attribute value from new values property

[talldan]

Backports the changes from WordPress/gutenberg#58596.

The gutenberg PR changes the data structure of the overrides data to allow it to be more flexible for future features.

Note that it might be best to commit this around the same time as gutenberg package updates land in core so that the block changes and core code changes are synchronized.

Trac ticket: https://core.trac.wordpress.org/ticket/60456

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[gziolo]

@talldan, when you are done with all the backports, it would be great to add some basic PHPUnit tests covering how Pattern Overrides work with block bindings. It could be on the _block_bindings_pattern_overrides_get_value level to start with. However, more useful would be to have some integration test that runs do_blocks( '<!-- wp:block {"ref":18,"overrides":{"AcqHKe":{"text":[1,"Hello"]}}} /-->' ) (I had the old format in my local instance).

[youknowriad]

commit https://core.trac.wordpress.org/changeset/57585