Accessibility: Replace `h4` with `h2` on settings modal titles

[sejas]

• Closes https://github.com/WordPress/wordpress-playground/issues/618

Description

It changes the header level on modal settings for create-block/wasm-demo block.

Testing instructions

• In a terminal run:
  • yarn install
  • composer install
  • yarn wp-env start # to start the main server
• In a new terminal run:
  • cd source/wp-content/themes/wporg-wasm/src/wasm-demo
  • nvm use v14.21.3
  • yarn install
  • yarn start # to start the wasm demo block
• Go to /wp-admin
• Activate the Wasm theme if it's not active yet.
• Create a new page
• Embed the block wasm demo
• View the forntend page
• Click on the settings button
• Inspect the titles Select a theme and Add plugins
• Observe they are h2 instead of h4
• I couldn't reproduce the issue with axe chrome extension

Screenshot

Before

After