[Stats Refresh] correct Posting Activity data

[ScoutHarris]

Fixes #11763

So, the limit parameter bit us again. An unspecified limit defaults to 10. limit: 0 doesn't return all. StatsPostingStreakInsight.queryProperties uses 5000, so I set the limit to 5000. And voila, we have a year's worth of data once again.

To test:

• Go to Stats > Insights > Posting Activity for an active site.
  • Tip: Hogwarts is a good one.
• Select View more.
  • The Insights card and details view use the same data, so verifying details should do it.
• Compare the data with Calypso (https://wordpress.com/stats/insights/<site_url>).

Calypso for Hogwarts:

Insights card:

Details:

Update release notes:

• If there are user facing changes, I have added an item to RELEASE-NOTES.txt.

[ghost]

@ScoutHarris : looks good to me!