CI: Update GitHub Actions to latest versions

CI: Update GitHub Actions to latest versions

Update action versions and organizations:
- gerrit-review-action: v0.8 -> v0.10.0 (SHA verified)
- Organization: lfit/* -> lfreleng-actions/*
- setup-python: @v5 -> @v5.4.0 (pin exact version)

Add Dependabot configuration for automated dependency updates:
- github-actions: weekly updates
- pip: weekly updates

Verified action SHAs:
- lfreleng-actions/gerrit-review-action@6d2e00dfd3173cd9a36d11350c8fba44731c7b4e (v0.10.0)
- lfreleng-actions/checkout-gerrit-change-action@54d751e8bd167bc91f7d665dabe33fae87aaaa63 (v0.9)
- actions/setup-python@v5.4.0 (42375524e23c412d93fb67b49958b491fce71c38)

The actions have moved from lfit organization to lfreleng-actions.
Using latest versions from builder-new reference implementation.

Change-Id: I96ec5eac5352d061de56938fb304f6cf07107f32
Signed-off-by: Anil Belur <abelur@linuxfoundation.org>