This moves the wiki pages that have a final destination under docs/postmortems/ as planned in [flutter.dev/go/migrate-flutter-wiki-spreadsheet](https://docs.google.com/spreadsheets/d/1x65189ZBdNiLRygpUYoU08pwvXD4M-Z157c6pm8deGI/edit?usp=sharing)
It also adds the team labels to the label bot for future PRs.
Changes to the content were only updating cross links, or links to refer to the main branch rather than master.
Remaining links to the wiki will be updated once all other pages have finished moving, they still work in the meantime.
Part of https://github.com/flutter/flutter/issues/145009
