Add upgrade notes for the fix on group updates

📖 What does this MR do?

These are the accompanying upgrade notes for #419289 (closed).

In short words:

• We created a fix regarding group path updates that contain NPM packages. This fix required a new database index.
• Due to the nature of that new database index, we needed to following timeline:
  1. Index created
  2. ANALYZE packages_packages; executed.
  3. Index can be used (the fix can be applied).
• To have more control on that timeline, we used a feature flag (see the discussion here). The idea is basically:
  1. In milestone m, introduce the index and the feature flag. We did this in %16.3
  2. In milestone m+1, remove the feature flag. We did this in %16.4
• gitlab.com is not affected by this since we could wait the ANALYZE execution before rolling out the feature flag.
• self-managed can be impacted. If they follow the releases: 16.3.0 -> 16.4.0, there are high chances that the ANALYZE will happen between the index creation and %16.4.

The risk here is for self-managed that will upgrade their setup "skipping" %16.3. For example, going from %16.2 to %16.4.

To mitigate this, this MR introduces upgrade notes that described that an ANALYZE query is required for %16.4.

⛓ Related issues

#419289 (closed)

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