feat(richtext-lexical)!: improve floating select menu Dropdown classNames

[AlessioGr]

Description

• Added extra unique class name for Dropdown button
• Fix incorrect aria label
• Fix incorrect className for Dropdown caret

Breaking:

• Dropdown component has a new mandatory sectionKey prop

• I have read and understand the CONTRIBUTING.md document in this repository.

Type of change

• Chore (non-breaking change which does not add functionality)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Change to the templates directory (does not affect core functionality)
• Change to the examples directory (does not affect core functionality)
• This change requires a documentation update

Checklist:

• I have added tests that prove my fix is effective or that my feature works
• Existing test suite passes locally with my changes
• I have made corresponding changes to the documentation