Use "current" in update URLs instead of CORE_COMPATIBILITY to retrieve all future updates

Problem/Motivation

Because of #2807145: [policy, no patch] Allow contrib projects to specify multiple major core branches, Drupal 9 may need to retrieve modules that start with 8.x-. The update module is not currently aware that modules starting with 8.x can be compatible with Drupal 9.

Proposed resolution

1. Update requests should no longer request information from URLs that have \Drupal::CORE_COMPATIBILITY(8.x) as the final path segment. This should be replaced with 'current'. For example, a request that previously requested https://updates.drupal.org/release-history/paragraphs/8.x will now request https://updates.drupal.org/release-history/paragraphs/current
2. The contents of /current has a somewhat different structure than what was returned by requests to /8.x, The update module and its tests will need refactoring to reflect those changes.
   • The following tags are no longer provided, the information should be parsed from the version tag instead: version_major, version_minor, version_patch, version_extra
   • recommended_major (which differed depending on the API requested) may be replaced with recommended_branches, which will be provided by Drupal.org if needed. Currently recommended_major is dropped
   • supported_majors replaced with supported_branches, which includes core compatibility for non-semver version numbers.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet