Issue #3630644: Provide default configuration instead of hardcoded defaults

• Ship config/install/gmap_polygon_field.settings.yml with the defaults of all settings, the same values as the hardcoded ones before.
• Existing sites keep their settings as they are, no update adds the missing ones. A post update would be reverted by the configuration import on sites that deploy the configuration from code.
• Warn in the status report when some settings are missing, with a link to the settings page. hook_runtime_requirements() on Drupal 11.3+, the legacy hook_requirements() on older versions.
• Read the settings in ConfigService without defaults, remove ConfigService::set() and the hardcoded defaults of the map script. A map with missing view settings shows the whole world, an empty or missing style setting falls back to the default of Google Maps.
• Use #config_target in the settings form, the config tag invalidation is not needed any more, the config object exists since install.
• Document the update in README.md.