chore: Update dependabot.yml template (#1813) (#6047)

* chore: Update `dependabot.yml` template not to touch pip dependencies
Source-Link: https://togithub.com/googleapis/synthtool/commit/f961eb0fe51109238128055897ccba1b70dbd804
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-java:latest@sha256:af2eda87a54601ae7b7b2be5055c17b43ac98a7805b586772db314de8a7d4a1d

Author: gcf-owl-bot[bot]