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
