Upgrade to @gitlab/eslint-plugin@6

This issue tracks updating @gitlab/eslint-plugin to version 6, which includes various improvements to our linting of Vue files, via eslint-plugin-vue. See https://github.com/vuejs/eslint-plugin-vue/releases/tag/v7.0.0 for details of what's been added/improved.

Implementation plan

• Upgrade to @gitlab/eslint-plugin@6
• For small/minor violations, either fix them, or line-disable the rule for those instances
• For large/major violations, disable the rule in .eslintrc.yml. If there are any of these, create a follow-up issue to address these and eventually remove the override.