Explore centralizing Dependabot configuration and shared-workflows SHA updates across keras-team repos

[jeffcarp]

Context

From @laxmareddyp in keras-team/keras#23768 (comment):

Would Dependabot help solve this? On a related note, our Dependabot configuration is currently duplicated across all our repositories—could we centralize it into shared-workflows to manage it across the entire Keras ecosystem from a single place?

Currently:

1. .github/dependabot.yml is duplicated across keras-team/keras, keras-team/keras-hub, and other keras-team repositories (with nearly identical github-actions and pip update schedules/groups, plus repo-specific ignore lists).
2. Caller workflows in keras-team repos pin both uses: keras-team/shared-workflows/.github/workflows/<workflow>.yml@<sha> and with: ref: '<sha>', which must be kept in sync when updating to a new shared-workflows revision.

Goals / Investigation

• Evaluate whether Dependabot (or an automated sync workflow in shared-workflows) can manage/sync .github/dependabot.yml across keras-team repositories from a single source of truth while preserving repo-specific package ignore entries.
• Evaluate how to keep uses: ...@<sha> and with: ref: '<sha>' automatically in sync when bumping shared-workflows versions (or whether inputs.ref can be derived/checked automatically).