Add github-actions ecosystem to Dependabot (sync workflow action pins)

[Chris-Wolfgang]

Closes the corresponding "CI/CD: Add the github-actions ecosystem to Dependabot" maintenance issue in this repo.

Summary

Adds the github-actions ecosystem block to .github/dependabot.yml so Dependabot keeps third-party action pins in workflow files up to date (currently it only manages NuGet packages).

Change

Appends the canonical block to dependabot.yml:

  # Keep third-party GitHub Actions pinned in workflows up to date.
  - package-ecosystem: "github-actions"
    directory: "/"
    schedule:
      interval: "weekly"
    open-pull-requests-limit: 10
    labels:
      - "dependencies"
    groups:
      github-actions:
        patterns:
          - "*"

Block matches the canonical version live in In-memory-Logger. Weekly schedule + grouped PRs to keep noise low; dependencies label so PRs are filterable.