testing/drone: upgrade to 2.3.1 and modernize
https://github.com/drone/drone/releases/tag/v2.3.1
agent and controller are removed https://github.com/drone/drone/blob/master/CHANGELOG.md#breaking
Edited by Andy Postnikov
https://github.com/drone/drone/releases/tag/v2.3.1
agent and controller are removed https://github.com/drone/drone/blob/master/CHANGELOG.md#breaking