Consolidate `label_select` on boards

What does this MR do?

This change affects the Epic and Issues boards creation/edit form.

A small MVC for: #292103 (closed). Our aim here is to do away with label_select/base.vue as it was only used in a single place i.e. the board_scope.vue component. In this effort we aim to re-use the new label_select_vue component which is used more widely in the application.

Screenshots (strongly suggested)

Before	After

How to test:

Issues

1. Navigate to Issues > Boards
2. Either select "Edit Board" or "Create new board"
3. See label select inside the board scope form

Epics

1. Enable Epic Boards feature flag echo "Feature.enable(:epic_boards)" | rails c
2. navigate to Epics > Boards
3. Either select "Edit Board" or "Create new board"
4. See label select inside the board scope form

Does this MR meet the acceptance criteria?

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Security

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• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
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