What?

Closes #69029

Removes the 'Custom' option from the FontSizePicker's select dropdown while maintaining custom font size functionality through the settings icon button.

Why?

The current implementation includes a 'Custom' option in the font size dropdown when there are more than 5 font sizes. This creates two issues:

• Semantically incorrect: A select/listbox control should contain values, not interactive controls that change the UI
• Accessibility violation: When using keyboard navigation, reaching the 'Custom' option unexpectedly changes the entire UI context and loses focus
• Redundant functionality: The same custom size feature is already available through the settings icon button

Testing Instructions

• Use a theme with more than 5 font sizes (e.g., modify Twenty Twenty-Four's theme.json to add extra font sizes)
• Open the block editor
• Select a Paragraph block
• Click on the Font size picker in the inspector panel and verify the "Custom" option is removed

Testing Instructions for Keyboard

• Use a theme with more than 5 font sizes (e.g., modify Twenty Twenty-Four's theme.json to add extra font sizes)
• Open the block editor
• Use Tab to focus on the font size and select dropdown
• Use arrow keys to navigate through options
• Verify that no unexpected UI changes occur

Screencast

Before -

After -