chore: Save weekly API diffs as markdown files (#10134)

* update to save diffs as md files

* test with test channels

* guard against case where the release commit didnt have caret ranges in package json but current does

the detected release commit (https://github.com/adobe/react-spectrum/commit/791377f0ad2093fb6ad36582e403289f13b0376b) doesnt have the carent range change from (https://github.com/adobe/react-spectrum/commit/81a95c9ea4abea02cb9f990a2e8ae3217029fbfd) and thus broke the script

* wrong slack env var

* wrong place to change

* ugh

* ugh pt2

* done testing