Support using the subhead multiple times on a page/post

[chrisvanpatten]

Description

Removes the supports.multiple = false setting on the subhead block so it can be used in multiple instances in a page/post/etc.

Fixes #8140.

How has this been tested?

Test by creating a page or post and adding the subhead block multiple times

Screenshots

Types of changes

Changes subhead so it can be used multiple times

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[karmatosed]

Let's close as for now we're going to keep this and have it remain single use.