Remove DB migrations that rely on rmp_serde

[LucasPickering]

In the past, Slumber encoded a lot of data in the sqlite DB as msgpack. This was a quick and dirty solution I used to easily shove stuff in there while I was iterating, and I didn't think too much about the impacts. #308 added a few migrations to move away from that. Unfortunately we still have rmp_serde in the dep tree because it's needed for the data migrations. As users upgrade to the version that included the migrations (should be 1.8.0), the need for that code will reduce over time. Eventually I would like to cut them out completely, which will eliminate a lot of code. It'll especially take out a good amount of serde codegen, which should help compile times a bit.

Removing the migrations will be a breaking change, so that will most likely be 2.0.0. Once the migrations are removed, anyone on <1.8.0 will have to upgrade to something >=1.8.0 <2.0.0 before going to 2.0.0. We'll need a small workflow to provide error messages if someone tries to make the big jump. I think a good waiting period before 2.0.0 would be about 6 months, so ~February 2025.