Expand dependabot alert count checks to all repos

[timmc-edx]

For https://2u-internal.atlassian.net/browse/SEG-78 (private ticket) we've added Dependabot alert checks to edx-repo-health. However, they only run on 55 of the ~300 repos due to permissions issues. We need to adjust the GitHub token that is used for that job. This might mean changing the teams that that service user is a member of, or switched to a fine-grained token that includes Dependabot access. (This is probably preferable anyhow, as it would allow read-only in a way that the classic tokens do not.)

[robrap]

Note: This will be marked with Backlog once edx/edx-arch-experiments#251 is complete and we have the new Backlog field.

[timmc-edx]

(May get picked up by SWG rather than Arch-BOM.)

[jristau1984]

openedx/axim-engineering#738 seems to have made at least some progress here. What else is left to make this happen? If all edx repos switched to read-only access, would it enable the final part of this automatically?