Fixed bug in v20 CampaignBudget resource.

- Added support for Google Ads API v20.

- Removed support for Google Ads API v17.

- Added support for Google Ads API v19_1.

- Updated upload_enhanced_conversions_for_leads example to use session attributes.

- Added support for Google Ads API v19.

- Removed support for Google Ads API v16.
- Removed feed-related examples.
- Updated PMax campaign creation examples to use brand guidelines.
- Updated add_shopping_product_ad example to remove usage of enhancedCpc.

- Fixed bug in Google::Ads::GoogleAds::V18::Resources::CampaignBudget.

- Added support for Google Ads API v18.

- Updated example get_all_disapproved_ads to set returnTotalResultsCount
  on new searchSettings request field.

- Added support for Google Ads API v17_1.

- Removed support for Google Ads API v15.

- Added support for Google Ads API v17.

- Updated examples to remove usage of the page size parameter in Search
  requests.

- Added support for Google Ads API v16.1.

- Removed support for Google Ads API v14.
- Removed examples: approve_merchant_center_link, reject_merchant_center_link,
  remove_flights_feed_item_attribute_value, update_flights_feed_item_string_attribute_value,
  add_real_estate_feed, add_flights_feed.

- Added support for Google Ads API v16.0.

- Updated examples to remove references to extensions.
- Added support for a flag on the client that allows making requests
  without a developer token. This is in preparation for the pilot
  that uses Google Cloud orgs for API Access levels.
- Added local_services_lead.contact_details and
  local_services_lead_conversation.message_details.text to the fields
  to mask from logs.