Let dependabot bump the Rust toolchain

The `rust-toolchain` ecosystem updates the `channel` in `rust-toolchain.toml`, which is now the only place the Rust version is written down. Weekly with the usual 14-day cooldown, so the `.1` point release has usually landed by the time the PR opens.

.github/dependabot.yml

@@ -1,3 +1,4 @@
+# Copyright 2025, 2026 Element Creations Ltd.
 # Copyright 2025 New Vector Ltd.
 #
 # SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
@@ -43,6 +44,17 @@ updates:
     cooldown:
       default-days: 14
 
+  # Rust releases every six weeks; the cooldown lets the .1 point release land first.
+  - package-ecosystem: "rust-toolchain"
+    directory: "/"
+    labels:
+      - "A-Dependencies"
+      - "Z-Deps-Backend"
+    schedule:
+      interval: "weekly"
+    cooldown:
+      default-days: 14
+
   - package-ecosystem: "github-actions"
     directory: "/"
     labels: