Abstract Apollo Cache update logic

What does this MR do?

Resolves #35074 (closed) by moving Apollo cache update logic to utilities for various design management components:

• ee/app/assets/javascripts/design_management/components/design_notes/design_discussion.vue
• ee/app/assets/javascripts/design_management/pages/design/index.vue
• ee/app/assets/javascripts/design_management/pages/index.vue

2 new utility files (specific to design management) have been added:

• cache_update.js - intended to hold code that directly updates Apollo cache
• errors.js - now that we're shifting updates to Apollo out of components (into apollo.js), errors are handled both within these abstractions and in components (usually when handling @error events in Apollo components).

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team