Update docs to use `migration_table` instead of `default_migration_table`.

[MasterOdin]

On a side note, the docs still says default_migration_table but I think that's deprecated since 0.13.

Originally posted by @zkwbbr in #2264

[IamMutevu]

Using 0.14 and it appears to still be using default_migration_table. I guess the docs is still okay unless I am missing something

[MasterOdin]

Yeah, we never removed the functionality so it still works, but I plan to, but it'd be good to update the docs first to give people more lead time to do the right thing.

Granted, there's always a warning that gets printed when using default_migration_table to tell people to use migration_table.

[IamMutevu]

That's noted. Thanks for the heads up.