Stop running Dependabot alongside Renovate

Both were updating `github-actions` daily, so both proposed the same bumps.
actions/setup-python v7 arrived twice within an hour: Renovate as #5575, Dependabot
as #5576.

Renovate is the one to keep. It pins actions to digests via
`helpers:pinGitHubActionDigests`, so master carries
`actions/setup-python@5fda3b9... # v7`, while Dependabot proposes a bare `@v7` and
would undo that pinning. Renovate's bumps also merge on a branch once CI has run
them, whereas Dependabot's sit outside every automerge rule here and need a human
for each one.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SEH3vxYSQ5SV4N5z61eyGT

.github/dependabot.yaml

@@ -1,11 +0,0 @@
-# SPDX-FileCopyrightText: 2022 Marko Weltzer
-#
-# SPDX-License-Identifier: AGPL-3.0-or-later
-
----
-version: 2
-updates:
-  - package-ecosystem: "github-actions"
-    directory: "/"
-    schedule:
-      interval: daily