rusqlite yanks have broken our minimal-versions CI

Versions 0.26.0 and 0.26.1 of rusqlite have been yanked, which means that our minimal-versions CI now uses 0.26.2. But 0.26.2 includes an edition = 2021 requirement, which means that it now requires 1.56 or later.

Here are our options:

• We could move our MSRV to 1.56.
• We could downgrade to rusqlite 0.25.4. (This version of rusqlite only supported time 0.2, however, which would require us to hack around that.)
• We could ask the rusqlite people to revert their MSRV change. Done in https://github.com/rusqlite/rusqlite/issues/1056
• We could migrate to a different database.