Graduate __experimental_woocommerce_blocks_checkout_update_order_meta interface to stable.

[nerrad]

This PHP action was added in January 2021. It is invoked when a draft order has been created or updated from the Cart (within the StoreAPI) and is now ready for extensions to modify the metadata.

This has been in use enough (current consumer includes Woo Subscriptions) that we should be able to graduate it from being experimental.

Things to do:

• Introduce a new action with the experimental prefix removed.
• Deprecate the old action (so it will still fire, but have it throw a deprecated developer notice pointing to the new name.
• Update inline code documentation.
• Update any other documentation.
• Check this action off in the list