The problem

Currently, TableDiff contains a list of dropped foreign key constraints. This is wrong for two reasons:

1. Dropping a constraint doesn't require knowing its full definition. The name is sufficient.
2. The name is also necessary, but constraint names are optional, so the definition isn't guaranteed to contain a name.

This way, we get what we don't need and we don't get what we need.

Manifestations in the code

1. In some cases, unnamed dropped foreign key constraints will lead to invalid SQL:

   dbal/src/Platforms/AbstractPlatform.php

   Lines 1372 to 1374 in e2e36dd

   	foreach ($diff->getDroppedForeignKeys() as $foreignKey) {
   	$sql[] = $this->getDropForeignKeySQL($foreignKey->getQuotedName($this), $tableNameSQL);
   	}

2. In some cases, they will be silently ignored:

   dbal/src/Platforms/SQLitePlatform.php

   Lines 933 to 938 in 6aa4070

   	foreach ($diff->getDroppedForeignKeys() as $constraint) {
   	$constraintName = $constraint->getName();
   	if ($constraintName === '') {
   	continue;
   	}

3. In another case, this design requires a weird back-and-forth translation in order to satisfy the API:

   dbal/src/Schema/SQLiteSchemaManager.php

   Lines 52 to 59 in 6aa4070

   	public function dropForeignKey(string $name, string $table): void
   	{
   	$table = $this->introspectTable($table);
   	$foreignKey = $table->getForeignKey($name);
   	$this->alterTable(new TableDiff($table, droppedForeignKeys: [$foreignKey]));
   	}

   The foreign key constraint definition is extracted from the table definition and is then passed alongside the table definition itself only to satisfy the API. Subsequently, the definition will be discarded, and only the name will be used.

Proposed solution

Instead of containing a list of dropped foreign key constraints, the diff should contain a list of their names.

Scope of the change

We cannot modify production code, because:

1. The constraints being dropped aren't guaranteed to have a name (see Deprecate unnamed foreign key constraints in $droppedForeignKeys of TableDiff #7143).
2. The names themselves aren't guaranteed to be valid UnqualifiedNames until 5.0.

The test is modified to demonstrate the proper use of the API.

Future scope

Likely, the same logic applies to all other dropped objects in the diff. I'd prefer to rework this logic object by object in order to minimize the diffs.