Migrate code_block_bubble_menu.vue to GlCollapsibleListbox

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What does this MR do and why?

One dropdown in app/assets/javascripts/content_editor/components/bubble_menus/code_block_bubble_menu.vue is being migrated to one of the new dropdown components(GlCollapsibleListbox)

References

Related Issue - #521412 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to the Content editor.
2. Click on More Options and select code block.
3. The dropdown to select language is the dropdown which is being migrated.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #521412 (closed)