Update s2n-quic submodule weekly through dependabot (#3053)

Use dependabot to update the `s2n-quic` submodule weekly (every Monday).
This is to avoid having to do this effort manually, and to guarantee
that none of the `s2n-quic` proofs break due to changes in Kani.

I've tested this in my own fork, and it seemed to work correctly.

.github/dependabot.yml

@@ -9,3 +9,10 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+
+  - package-ecosystem: "gitsubmodule"
+    directory: "/"
+    allow:
+      - dependency-name: "tests/perf/s2n-quic"
+    schedule:
+      interval: "weekly"