Suggest enabling dependabot action updates

[elopez]

Newer versions of the slither action can bring new features and support for newer slither releases, so it's good to stay up to date. We should suggest enabling this feature on users' repositories to help them notice new releases.

https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot