Upgrade flux terraform provider

[andrasmaroy]

With 1.0.0 of the flux provider the bootstrap process was drastically changed, making a migration necessary for upgrading the provider. There's a guide for the process.

[andrasmaroy]

Not feasible because flux cannot be bootstrapped with tags (which is fine), but in the provider that is tied to referencing the target git repository as well.

See fluxcd/flux2#2726