Return &mut Self from the migrator set_ methods

[nipunn1313]

This allows the migrator methods to be chained.

Fixes #1788

[abonander]

@nipunn1313 sorry about how long it took me to look at this, but this won't make this example work because you need something that takes self and returns Self (i.e. not a reference) to result with an owned Migrator.

It would have to be something like

pub const fn with_ignore_missing(self, value: bool) -> Self

I labeled this as breaking because while it seems innocuous enough--in most cases &mut Migrator will coerce to &Migrator just fine--I can conceive of a situation where this would break code that compiled before: https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=2b874b42707b732a3cd20690a64d5290