chore: Switch dependency updates from Dependabot to Renovate

Matches celo-org convention: extend the org-wide preset at
celo-org/.github:renovate-config (config:recommended + 7-day
minimumReleaseAge + OSV vulnerability alerts + earlyMondays
schedule). Same layout as the sibling repo celo-org/blind-threshold-bls-wasm.

Removes .github/dependabot.yml so we don't run both tools in
parallel. config:recommended covers both cargo and github-actions,
so this closes the cargo dependency-update gap without regressing
github-actions coverage.

Author: palango

.github/dependabot.yml

@@ -0,0 +1,6 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": [
+    "local>celo-org/.github:renovate-config"
+  ]
+}