Contact Form Block: Converting classic block (with contact form) to regular blocks converts it to a proper block

#### Steps to reproduce

- Start with a `classic block`
- Add a contact form in it
- Convert this block to regular blocks using the 3 dots at the top right of the block

#### What is expected

The contact form becomes a shortcode block

#### What is suggested

The contact form becomes a proper contact form block.

#### Notes

It does not feel to be a bug, hence an enhancement label.

#support #user-report

Reproduce the conversion from a classic block containing a contact form to regular blocks, then trace the block-conversion entry point used by the three-dot menu. The work is done when the contact form converts to a proper contact form block rather than a shortcode block, matching the expected behavior in the issue.

Written by the indexing model from the issue text.