add dependabot by default in the OWNERS file

[chmouel]

in tkn pac repo, if we detect installing on Github it would be a nice touch to create a OWNERS file with dependabot to let it allow to run the PR...

cf, issue reported by @vdemeester

[chmouel]

if it's a github repo and there is no OWNERS file at the root directory then add one with no question :

# This allows the dependabot user to run Pipelines as Code automatically
# You can add more users in there or remove it.
- reviewers:
  -  dependabot
 

[chmouel]

thinking a bit more about it, I don't think we should do this by default! it's up to the user to configure one and i think it's called dependabot[bot]