Rename the `blocklyTreeIconClosed` CSS class to `blocklyToolboxCategoryIconClosed`

[BeksOmega]

Rename any references to blocklyTreeIconClosed to blocklyToolboxCategoryIconClosed.

This change should be made against the rc/v12.0.0 branch. This should be marked as a breaking change.

To work on this issue, comment below asking to be assigned.

[YashrajKupekar17]

@BeksOmega I would like to work on this issue. please could you assign me this issue?

[BeksOmega]

Go for it @YashrajKupekar17 =) Let me know if you have any questions!

[harsh-791]

@BeksOmega If no one is working on this, I would llike to work on this

[BeksOmega]

Go for it @harsh-791

[harsh-791]

@BeksOmega Can u review this PR and tell me if any changes need to be done