Handle cache busting scenario when note author detail changes

What does this MR do?

Before when a note author which is part of a discussion has its detail updated (e.g. name change, username change), the discussion cache wont be busted.

To ensure that we show updated information when that happens, the author's cache key is now included in Note#post_processed_cache_key which we call in Discussion#cache_key.

This way, when the author is updated, the discussion cache will be invalidated as well.

Does this MR meet the acceptance criteria?

Conformity

• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)

Related to #336220 (closed)