[feature] Add support for Ubuntu 26.04

[nemesifier]

Is your feature request related to a problem? Please describe.

Add support for Ubuntu 26.04.

Describe the solution you'd like

CI needs to support it, docs and ansible galaxy meta info need to be updated to reflect this.

[openwisp-companion]

This issue has been automatically assigned to @nemesifier who opened PR #634 to address it. 🎯