Configure dependabot to check github-actions updates

[Goooler]

Some actions based on Node 16 are deprecated.

See

• https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20
• https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot#example-dependabotyml-file-for-github-actions

[Goooler]

Actions updates have been sent to my fork, see https://github.com/Goooler/actions-runner/pulls?q=is%3Apr+is%3Aopen+label%3Agithub_actions