Fix a clippy warning in datafusion-sqllogictest

[mbrobbel]

Which issue does this PR close?

error: use of a disallowed method `tokio::task::spawn`
  --> datafusion/sqllogictest/src/engines/postgres_engine/mod.rs:93:27
   |
93 |         let join_handle = tokio::spawn(async move {
   |                           ^^^^^^^^^^^^
   |
   = note: To provide cancel-safety, use `SpawnedTask::spawn` instead (https://github.com/apache/datafusion/issues/6513)
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#disallowed_methods
   = note: `-D clippy::disallowed-methods` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::disallowed_methods)]`

error: could not compile `datafusion-sqllogictest` (lib) due to 1 previous error

Rationale for this change

#14483 (comment)

What changes are included in this PR?

Use SpawnedTask::spawn instead of tokio::task::spawn.

Are these changes tested?

CI.

Are there any user-facing changes?

No.

[alamb]

Thanks (again) @mbrobbel