chore: enable github action auto update in dependabot.yml

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.98%. Comparing base (ffa2b90) to head (9819231).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #337 +/- ## ======================================= Coverage 93.98% 93.98% ======================================= Files 30 30 Lines 2924 2924 Branches 649 649 ======================================= Hits 2748 2748 Misses 148 148 Partials 28 28 ``` | [Flag](https://app.codecov.io/gh/openedx/opaque-keys/pull/337/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openedx) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/openedx/opaque-keys/pull/337/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openedx) | `93.98% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openedx#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.