Increment the stored notesCount

What does this MR do?

This MR fixes up a missing reactive feature for !16416 (merged).

When adding a new discussion or commenting on an existing discussion, the total notes count on the design item is not updated.

This MR resolves that by incrementing the value stored in the Apollo Cache for the design in question in the update handler for each mutation (adding a discussion and replying to a discussion).

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

Performance 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