Allow scripts to match the migration starting version

[oliverfoster]

Sometimes a migration will add a default and then mutate a default when extending over a multiple versions.

For example, the Hot Graphic instruction value went from "" (empty string) to "Select the icons to find out more" in v6.6.0.

It should be possible to check if the migration started from before the initial introduction of the empty string or after, so as to be able to optionally mutate the default.