fix: freeze lxd to 5.20/stable

[gruyaume]

Description

Charmcraft currently does not work with the latest lxd version (5.21/stable). This prevents multiple PR's being merged. This change freezes lxd to 5.20/stable.

Reference

• https://github.com/canonical/charmcraft/issues/1640

Checklist:

• [ ] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests that validate the behaviour of the software
• [ ] I validated that new and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules
• [ ] I Have bumped the version of the library