Update Drupal media kit pages

Problem/Motivation

Some pages in the Drupal Media Kit seem outdated. Links to several recent DrupalCon Flickr photo pools are missing. The last DrupalCon mentioned on this page → is Amsterdam 2019.

Proposed resolution

Add links to recent DrupalCon Flickr pools to https://www.drupal.org/about/media-kit/social-media → and https://www.drupal.org/about/media-kit/photography → .

Remaining tasks

1. Gather links to recent DrupalCon Flickr pools
2. Add missing links to https://www.drupal.org/about/media-kit/social-media →
3. Add missing links to https://www.drupal.org/about/media-kit/photography →