Configre dependabot PRs schedule

[uzabanov]

Background

With the recent cost reduction efforts we have configured the PR clusters to be destroyed at 00:00 UTC and then created at 08:00 UTC. This means that if dependabot PRs a dependency bump during the night, that PR checks would fail as there is no PR cluster to run tests.

It seems that one can configure dependebot when to perform its check. Lets make use of it to ensure that PRs are created while there are PR clusters around.