add dependabot to update verif/core-v-verif

Created by: cathales

This PR adds a dependabot configuration file which will have the following behavior:

• Each monday, if the verif/core-v-verif submodule hash is outdated (compared to the current master branch of core-v-verif), it opens a new PR updating the submodule, with the Component:Verif label.
• As a new PR is open, the dashboard will run and tell us in a comment if it breaks things or not.
  • If it passes, we can merge it by simply clicking the green button.
  • If it fails, we know that we have things to get fixed.
• If there was already such a PR which is itself outdated (ie. if we did not take any actions in a week and CVV has changed in the meantime), dependabot closes it and opens a new one.

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PS: we should maybe introduce this line into the readme:

git config --global submodule.recurse true

So that subsequent git pull’s update CVV submodule too.