Extensions can no longer define a 'major' value in their .info.yml files

The Update Manager used to allow extensions to define a 'major' key in their .info file to define what major version to use for comparisons and recommendations. This was added for projects not hosted on Drupal.org (with its allowed version strings that define the major version).

This rarely used feature is no longer suppported. Any extension that interfaces with the Update Manager should be using a version string that defines a major version implicitly (either the with legacy Drupal version numbers "like 8.x-1.3" or SemVer version strings like "2.1.3").