fix: findRoots should get the defined primary key column

[ilyasfoo]

Description of change

Previously findRoots assumes the primary column of an entity to be named id

typeorm/src/repository/TreeRepository.ts

Lines 35 to 37 in a5eb946

	const parentPropertyName = this.manager.connection.namingStrategy.joinColumnName(
	this.metadata.treeParentRelation!.propertyName, "id"
	);

This PR will instead use the column defined by entity class

Fixes #6948, #2361

Pull-Request Checklist

• Code is up-to-date with the master branch
• npm run lint passes with this change
• npm run test passes with this change
• This pull request links relevant issues as Fixes #0000
• There are new or updated unit tests validating the change
• The new commits follow conventions explained in CONTRIBUTING.md

[pleerock]

Thank you!