1704 postrelease checklist update

[benlk]

Changes

This pull request makes the following changes:

• Updates the release checklist template with new items from 0.6.4's Largo 0.6.4 release checklist #1704
• Adds pain points from the 0.6.4 release cycle
• Adds checklist items for things that we may have accidentally missed in 0.6.4

Why

• To improve the release checklist, as part of checklist for releases #809's goal for a checklist for releases
• To update the checklist, in accordance with Largo 0.6.4 release checklist #1704

Testing/Questions

Questions that need to be answered before merging:

• Is there anything else that needs to be added?

Steps to test this PR:

1. Copy the markdown section of this doc into a draft GitHub issue and hit 'preview' to make sure it parses