Add Maven as supported Renovate manager (#136)

.github/renovate.json

@@ -1,7 +1,7 @@
 {
   "$schema": "https://docs.renovatebot.com/renovate-schema.json",
   "extends": ["github>bitwarden/renovate-config"],
-  "enabledManagers": ["bundler", "github-actions", "gradle"],
+  "enabledManagers": ["bundler", "github-actions", "gradle", "maven"],
   "packageRules": [
     {
       "groupName": "bundler minor",