[Docs] Include MaxMind license key configuration requirement for GeoIP database for 9.x versions

[GabiHausmann]

Please update the GeoIP database API documentation to include the same guidance that existed in earlier documentation versions regarding MaxMind commercial license keys.

The following information appears in the 8.15 docs but is missing or unclear in newer versions:

The MaxMind account ID requires a license key. Because the license key is sensitive information, it must be stored as a secure setting in Elasticsearch named `ingest.geoip.downloader.maxmind.license_key`. Only one MaxMind license key is allowed per Elasticsearch cluster. A valid license key must be present in the secure settings to download databases from MaxMind. The license key setting does not take effect until all nodes are restarted.

Previous documentation (8.15): https://www.elastic.co/guide/en/elasticsearch/reference/8.15/put-geoip-database-api.html

[mmahacek]

This info should also be added to https://www.elastic.co/docs/reference/enrich-processor/geoip-processor as a secure setting for commercial MaxMind users.