[8.x] Add dropConstrainedForeignId to Blueprint

[amir9480]

Hello.

This PR adds a new method dropConstrainedForeignId to the migrations which useful to drop columns made with foreignId.

---

A sample migration before this change:

class AddCategoryIdToPostsTable extends Migration
{
    public function up()
    {
        Schema::table('posts', function (Blueprint $table) {
            $table->foreignId('category_id')->constrained()->cascadeOnDelete()->cascadeOnUpdate();
        });
    }

    public function down()
    {
        Schema::table('posts', function (Blueprint $table) {
            $table->dropForeign(['category_id']);
            $table->dropColumn('category_id');
        });
    }
}

Same migration after this change:

class AddCategoryIdToPostsTable extends Migration
{
    public function up()
    {
        Schema::table('posts', function (Blueprint $table) {
            $table->foreignId('category_id')->constrained()->cascadeOnDelete()->cascadeOnUpdate();
        });
    }

    public function down()
    {
        Schema::table('posts', function (Blueprint $table) {
            $table->dropConstrainedForeignId('category_id');
        });
    }
}

[coatesap]

Nice work, this should make the down migrations of FK constraints a bit easier in Laravel.

It's a shame that it's still not a symmetrical migration though, which would be more intuitive. We now have:

// up
$table->foreignId(...)->constrained()
// down
$table->dropConstrainedForeignId(...)

Whereas this would be a more symmetrical version:

// up
$table->constrainedForeignId(...)
// down
$table->dropConstrainedForeignId(...)

Or even (if we could automatically detect the constraint):

// up
$table->foreignId(...)
// down
$table->dropForeignId(...)

And we've got a similarly annoying lack of symmetry in:

// up
$table->foreign('column_name')
// down
$table->dropForeign(['column_name'])

Where the down suddenly requires an array.

I'm not sure if it's possible to smooth out these inconsistencies whilst dealing with backwards compatibility, but I'm happy to help if anyone has ideas.