So, a new version of Technic Solder has been released. The version is 0.7.6, it's based on Laravel 6 and supports PHP 7.2+ (up to 8.0).
There are a bunch of improvements and bugfixes which should make everything less painful to work with.
The documentation at https://docs.solder.io/ has been updated to this new version, and the old one (v0.7, PHP 5.6 to 7.0) has been marked as unsupported.
If you're upgrading from Solder v0.7, remember to run
php artisan migrate after you install the new version.