feat: configure Dependabot Version Updates for workflows and actions

[Capstan]

Description

To address the outdated dependencies mentioned in #152 (since Renovate was rejected in favor of Dependabot in #154), we should configure Dependabot Version Updates for this repository.

Currently, Dependabot is not configured for version updates (there is no .github/dependabot.yml file). This means that while security alerts might be enabled, regular updates for stale dependencies are not automated.

Specifically, we need updates for:

1. GitHub Actions used in workflows and composite actions (to keep SHAs/versions up to date).
2. npm dependencies in the multi-approvers action (.github/actions/multi-approvers/package.json).

Proposed Solution

Add a .github/dependabot.yml configuration file to the root of the repository to enable weekly version updates.

Example .github/dependabot.yml:

version: 2
updates:
  # Enable version updates for GitHub Actions
  - package-ecosystem: "github-actions"
    directory: "/"
    schedule:
      interval: "weekly"

  # Enable version updates for npm dependencies in multi-approvers
  - package-ecosystem: "npm"
    directory: "/.github/actions/multi-approvers"
    schedule:
      interval: "weekly"