dependabot: use directories and use docker ecosystem

[v1v]

What does this pull request do?

Use dependabot for updating docker images stored in our internal docker registry using this.

Use directories to manage the dependencies for the GitHub workflows and composite actions.

Why

We already have secrets stored for accessing the internal docker registry. Use one dependency management tool with native support for GitHub actions and the GH secrets access control.

This has been already tested in the past in a sandbox repository and also in other GH internal repositories.

Actions

• Create Dependabot GitHub secret using CasC.

Related issues

See elastic/apm-agent-nodejs#4539 and elastic/apm-agent-python#2246

Checklist

• This is an enhancement of existing features, or a new feature in existing plugins
  • I have updated CHANGELOG.next-release.md
  • I have added tests that prove my fix is effective or that my feature works
  • Added an API method or config option? Document in which version this will be introduced
  • I have made corresponding changes to the documentation
• This is a bugfix
  • I have updated CHANGELOG.next-release.md
  • I have added tests that would fail without this fix
• This is a new plugin
  • I have updated CHANGELOG.next-release.md
  • My code follows the style guidelines of this project
  • I have made corresponding changes to the documentation
  • I have added tests that prove my fix is effective or that my feature works
  • New and existing unit tests pass locally with my changes
  • I have updated supported-technologies.md
  • Added an API method or config option? Document in which version this will be introduced
  • Added an instrumentation plugin? Describe how you made sure that old, non-supported versions are not instrumented by accident.
• This is something else
  • I have updated CHANGELOG.next-release.md