Editor: Hide Classic Block from inserter

[tyxla]

Trac ticket: https://core.trac.wordpress.org/ticket/65166

This PR brings the changes from the following Gutenberg PR to core:

WordPress/gutenberg#77911

Description

We're hiding the Classic block in the block editor inserter by default.

We're also introducing a new wp_classic_block_supports_inserter filter that allows overriding that behavior globally or per post.

See: WordPress/gutenberg#74242

Testing

• Start writing a new post
• Verify you don't see the Classic block in the inserter.
• Open an existing post that contains a classic block (or a <!-- wp:freeform --> block) and confirm the Classic block renders correctly and is editable.
• Put this in your code: add_filter( 'gutenberg_classic_block_supports_inserter', '__return_true' );. Alternatively, you can use this plugin to test it.
• Verify you see the Classic block in the inserter.

[github-actions]

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[mukeshpanchal27]

Add missing doc block for global