feat(renovate): Adds renovate config for mariadb-operator updates

[nicholaskuechler]

Adds renovate config for mariadb-operator updates.

I gave it a test run with:

docker run --rm -it \
  -v "$(pwd)":/usr/src/app \
  -e LOG_LEVEL=debug \
  renovate/renovate \
  --dry-run \
  --platform=local

And it looks like it's working:

         {
           "branchName": "renovate/mariadb-operator",
           "prNo": null,
           "prTitle": "Update MariaDB Operator to v25.8.4",
           "result": "error",
           "upgrades": [
             {
               "datasource": "helm",
               "depName": "mariadb-operator",
               "displayPending": "",
               "fixedVersion": "25.8.3",
               "currentVersion": "25.8.3",
               "currentValue": "25.8.3",
               "newValue": "25.8.4",
               "newVersion": "25.8.4",
               "newDigest": "aab019d3dfc876e2d14314bbeaf39e3ad5107ebe829e682f833a6fe7b97c0151",
               "packageFile": "operators/mariadb-operator/kustomization.yaml",
               "updateType": "patch",
               "packageName": "mariadb-operator"
             },
             {
               "datasource": "helm",
               "depName": "mariadb-operator-crds",
               "displayPending": "",
               "fixedVersion": "25.8.3",
               "currentVersion": "25.8.3",
               "currentValue": "25.8.3",
               "newValue": "25.8.4",
               "newVersion": "25.8.4",
               "newDigest": "017ef7cb13e81cee45dfd9bd261acce0260d54ea171b77b8383a8b79ffaeea1a",
               "packageFile": "operators/mariadb-operator/kustomization.yaml",
               "updateType": "patch",
               "packageName": "mariadb-operator-crds"
             }
           ]
         },