fix(coderd/database): ensure task name uniqueness (#20236)

This change ensures task names are unique per user the same way we do
for workspaces. This ensures we don't create tasks that are impossible
to start due to another task being named the same creating a workspace
name conflict.

Updates coder/internal#948
Supersedes coder/coder#20212

coderd/database/migrations/000380_task_name_unique.down.sql

@@ -0,0 +1 @@
+DROP INDEX IF EXISTS tasks_owner_id_name_unique_idx;