Enable Dependabot for GitHub Actions (#8778)

Our workflows use old GitHub Actions. For example, we use
`actions/checkout@v3` but `actions/checkout@v5` is the latest version:

https://github.com/google/flatbuffers/blob/599847236c35fa3802ea4e46e20e93a55d3a4a94/.github/workflows/build.yml#L33

https://github.com/actions/checkout/releases

How about enabling Dependabot? If we enable Dependabot, Dependabot
opens PRs that update old GitHub Actions.

Dependabot document:
https://docs.github.com/en/code-security/dependabot

Dependabot configuration document:
https://docs.github.com/en/code-security/dependabot/working-with-dependabot/dependabot-options-reference