Update instances of deprecatedCreateFlash with new createFlash

What does this MR do?

Replaces a deprecatedCreateFlash with createFlash for the following components:

• app/assets/javascripts/design_management/pages/index.vue
• app/assets/javascripts/design_management/mixins/all_designs.js
• app/assets/javascripts/design_management/components/upload/design_dropzone.vue
• app/assets/javascripts/design_management/pages/design/index.vue
• app/assets/javascripts/design_management/utils/cache_update.js

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #246743 (closed)