feat(k3s_upgrade): regenerate service files from templates instead of restoring backups (#474)

* feat(k3s_upgrade): regenerate service files from templates instead of restoring backups

The k3s_upgrade role previously restored backup service files after binary
upgrade, which meant any changes to extra_server_args or extra_agent_args
would not be applied during upgrades.

This change:
- Replaces backup restoration with template-based service file regeneration
- Reuses templates from k3s_server role via symlink
- Adds necessary defaults for template variables

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Aleksei Sviridkin <f@lex.la>