chore: explicitly specify maven central location for renovate-bot version lookups

[mpeddada1]

From #3372, the files .cloudbuild/directories aren't getting updated to use the latest java-shared-config version. From the renovate logs, there appears to be a difference in the registry urls used when looking up versions from maven central.

Registry used when an update to the latest version is successfully made to 1.12.0:

     {
        "datasource": "maven",
        "deps": [
          {
            "currentValue": "1.11.3",
            "currentVersion": "1.11.3",
            "datasource": "maven",
            "depName": "com.google.cloud:google-cloud-shared-config",
            "depType": "parent-root",
            "fileReplacePosition": 702,
            "fixedVersion": "1.11.3",
            "isSingleVersion": true,
            "packageName": "com.google.cloud:google-cloud-shared-config",
            "packageScope": "com.google.cloud",
            "registryUrls": [
              "https://maven-central.storage-download.googleapis.com/maven2/",
              "https://repo.maven.apache.org/maven2"
            ],
            "sourceUrl": "https://github.com/googleapis/java-shared-config",
            "versioning": "maven",
            "warnings": [],
            "updates": [
              {
                "bucket": "non-major",
                "newVersion": "1.12.0",
                "newValue": "1.12.0",
                "releaseTimestamp": "2024-11-13T20:18:15.000Z",
                "newVersionAgeInDays": 0,
                "registryUrl": "https://maven-central.storage-download.googleapis.com/maven2",
                "newMajor": 1,
                "newMinor": 12,
                "newPatch": 0,
                "updateType": "minor",
                "branchName": "renovate/com.google.cloud-google-cloud-shared-config-1.x"
              }
            ]
          },

For .cloudbuild/ files, the registryUrl defaults to https://repo.maven.apache.org/maven2:

      {
        "datasourceTemplate": "maven",
        "depNameTemplate": "com.google.cloud:google-cloud-shared-config",
        "deps": [
          {
            "currentValue": "1.11.3",
            "currentVersion": "1.11.3",
            "currentVersionAgeInDays": 43,
            "currentVersionTimestamp": "2024-10-01T21:54:00.000Z",
            "datasource": "maven",
            "depName": "com.google.cloud:google-cloud-shared-config",
            "fixedVersion": "1.11.3",
            "packageName": "com.google.cloud:google-cloud-shared-config",
            "packageScope": "com.google.cloud",
            "registryUrl": "https://repo.maven.apache.org/maven2",
            "replaceString": "  _JAVA_SHARED_CONFIG_VERSION: '1.11.3'",
            "sourceUrl": "https://github.com/googleapis/java-shared-config",
            "versioning": "maven",
            "warnings": [],
            "updates": []
          }
        ],
        "matchStrings": [
          "  _JAVA_SHARED_CONFIG_VERSION: '(?<currentValue>.+?)'"
        ],
        "packageFile": ".cloudbuild/graalvm/cloudbuild-test-b-downstream-kmsinventory.yaml"
      },

Explicitly specifying registryUrl to https://repo1.maven.org/maven2 worked in https://github.com/mpeddada1/forking-renovate-repro

[sonarqubecloud]

Quality Gate passed for 'gapic-generator-java-root'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[sonarqubecloud]

Quality Gate passed for 'java_showcase_integration_tests'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud