Fix Cherry Pick script to include all PRs

[getdave]

During the Beta 2 for WordPress 5.6, 22 PRs were missed by the automated cherry picking script

npm run cherry-pick

@youknowriad and I believe this is because by default the Github paginates its REST API results. It defaults to 30 items per day (see per_page). Therefore if the number of PRs to be commit exceeds this default then it will miss PRs outside the first "page" of results.

To avoid this happening we should do two things:

• update the release documentation to add a "confidence check" step following running of the cherry pick script to manually check that all relevant PRs were picked correctly.
• Update the cherry pick script to utilise pagination as per the Github API guidance.