Remove the old disabled Dependabot configuration

In the past (#143), Dependabot version updates had been used to
keep `cargo` dependencies up to date. This was removed in favor of
doing manual updates based on automatic reports from `cargo deny`
and the old `dependabot.yml` was kept but renamed to disable it and
point people to #144 to learn about the change. Since then,
Dependabot security updates, which are distinct from Dependabot
version updates, were enabled (see #1254), and later, Dependabot
version updates were reintroduced for GitHub Actions only (#1357).
At that point, there were two Dependabot-related YAML files: the
old disabled one, and the new one for GHA.

This removes the old one, explaining the situation in a comment in
the new one, including a link to #144.

While doing so, this also adjusts the YAML code style there, to
bring it in line with the style of most other YAML files in the
repository.

.github/dependabot.yml

@@ -1,9 +1,12 @@
 version: 2
 updates:
-- package-ecosystem: github-actions
-  directory: "/"
-  schedule:
-    interval: weekly
-  groups:
-    github-actions:
-      patterns: ["*"]
+  # We only use Dependabot *version* updates for GitHub Actions. Rust dependencies are checked via
+  # `cargo deny` and manually updated (see https://github.com/GitoxideLabs/gitoxide/issues/144), or
+  # by Dependabot *security* updates (which don't need the `cargo` ecosystem to be listed here).
+  - package-ecosystem: github-actions
+    directory: '/'
+    schedule:
+      interval: weekly
+    groups:
+      github-actions:
+        patterns: ['*']