ci: Dependabot configuration to update actions in workflows

[ScottBrenner]

Noticed the actions used in the workflows here are outdated, proposing a Dependabot configuration to update - reference https://docs.github.com/en/actions/security-guides/using-githubs-security-features-to-secure-your-use-of-github-actions#keeping-the-actions-in-your-workflows-secure-and-up-to-date

Resolves warnings on workflow runs e.g. https://github.com/cohere-ai/cohere-compass-sdk/actions/runs/25743559129

Node.js 20 actions are deprecated. The following actions are running on Node.js 20 and may not work as expected: actions/checkout@v4, actions/setup-python@v4.

Suggest enabling https://docs.github.com/en/code-security/dependabot/working-with-dependabot/about-dependabot-on-github-actions-runners#enabling-or-disabling-for-your-repository as well

[cla-assistant]

All committers have signed the CLA.

[ScottBrenner]

Hey @CelineWei-cohere spare a moment to review?