Resolve "i18n Broken - Boards blank state"

What does this MR do?

Fixes broken i18n translations for the boards empty state. Use s__ on all namespaced translations in app/assets/javascripts/boards/components/board_blank_state.vue.

Before

After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #64260 (closed)