Enable auto-merge on PRs for dependabot

[tangxifan]

Is your feature request related to a problem? Please describe.
Currently, the PRs created by dependabot still require code owners's review and approval.
However, this may eat a lot of time from engineers, since this project:

• integrates OpenFPGA and OpenSTA, check the correctness of the integration
• check compilation compatibility for the submodules. Correctness of OpenFPGA and OpenSTA should be already checked in their own regression tests

Describe the solution you'd like

• I have already enable auto-merge and remove any protection on main branch in github repo settings
• See how to enable this feature in the dependabot.yml

[alain-rs]

OpenSTA has moved. We can close this issue as the OpenFPGA dependabot is working.