Restrict Dependabot to direct dependencies (#719)

Avoid useless PRs like #716

Also, give Dependabot's toolchain updater a try.

Author: mlafeldt

.github/dependabot.yml

@@ -4,6 +4,14 @@ updates:
     directory: "/"
     schedule:
       interval: daily
+    allow:
+      - dependency-type: direct
     open-pull-requests-limit: 10
     target-branch: main
     labels: [auto-dependencies]
+  - package-ecosystem: rust-toolchain
+    directory: "/"
+    schedule:
+      interval: daily
+    target-branch: main
+    labels: [auto-dependencies]