Manually migrate heading block to the new syntax to avoid block validation errors

[Tug]

Fixes the following errors happening since the Heading block was updated in WordPress/gutenberg#16035

Block validation: Block validation failed for `core/heading` ({name: "core/heading", save: ƒ, category: "common", attributes: {…}, title: "Heading", …}).

Content generated by `save` function:

<h2 class="has-text-align-left">Meet your new best friends, Blocks</h2>

Content retrieved from post body:

<h2 style="text-align:left">Meet your new best friends, Blocks</h2>

To test:

Boot the app locally and check that the block validation errors are no more.

Edit: Fixes #1141

[SergioEstevao]

Thanks for this.