chore(deps): drop the Dependabot actions fallback now Renovate runs

[ExtraToast]

Why

.github/dependabot.yml stated its own exit condition:

# Renovate (renovate.json) is the primary dependency manager for JorisJonkers-dev
# repos. Dependabot is enabled only for GitHub Actions security updates as a
# belt-and-braces fallback; disable this file if Renovate covers actions in a
# given repo.

That condition is now met. Renovate is installed and enabled on the organization, and the shared preset at JorisJonkers-dev/renovate-config covers the actions manager twice over:

{ "matchManagers": ["github-actions"], "groupName": "github actions dependencies" }
{ "matchManagers": ["github-actions"], "pinDigests": true }

Keeping both means every action bump arrives as two PRs against the same file — one grouped and digest-pinned from Renovate, one single-dependency from Dependabot.

Scope

The Dependabot config declared exactly one ecosystem, github-actions, so nothing else is lost. Renovate additionally pins digests, which this config did not.

The same change already merged in github-workflows, gradle-conventions and kotlin-spring-commons. This repository and .github were missed in that pass; repo-template also carries the fallback but removing it there is a template-policy decision, since scripts/validate-templates.sh asserts the file exists.

Also in this branch

docs/REPO_SETUP.md listed dependabot.yml in the repository layout table. That row would have pointed at a file that no longer exists, so it now names only renovate.json. A grep for dependabot across the repository returns nothing after both commits.