Switch to native Dependabot

Previously Dependabot was a third-party service. Now that Dependabot
has been bought by GitHub, it's being integrated into GitHub itself as
a native service. To make use of the native version, the configuration
changes slightly, and it goes under a new directory.

.dependabot/config.yml

@@ -1,9 +0,0 @@
-version: 1
-
-update_configs:
-  - package_manager: "go:modules"
-    directory: "/"
-    update_schedule: "daily"
-  - package_manager: "github_actions"
-    directory: "/"
-    update_schedule: "daily"

.github/dependabot.yml

@@ -0,0 +1,12 @@
+version: 2
+
+updates:
+  - package-ecosystem: "gomod"
+    directory: "/"
+    schedule:
+      interval: "daily"
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "daily"