Update/block inserter headings

[gaambo]

Description

Changes the headings of block groups from spans to h2 to provide a semantic structuring. Related to #22859

How has this been tested?

• Open the block inserter - headings should look exactly as before the commit (same margin, font-size etc).
• Should be tested with a screenreader?

Screenshots

Types of changes

Bug fix, css-class is the same as before and styling is the same.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[gaambo]

sorry, used the wrong branch from a previous commit. gonna do it new based on master.