[9.x] Add ability to drop an index when modifying a column

[hafezdivandari]

-> We can already chain an index method on the column definition to add an index:

$table->string('name')->unique();

-> We could also modify a column to drop its attribute, e.g. set not null:

$table->string('name')->nullable(false)->change();

This PR adds the ability to drop an index (by its conventional name) when modifying a column:

$table->string('name')->unique(false)->change();