[SE-0417] Revise Task(executorPreference:) ownership semantics

[ktoso]

This is just a cleanup of a semantics section which previously showed what was unsafe, but actually isn't unsafe anymore since we fixed it.

This should not require a re-review, as it does not change API surface in any way, it's more of a bugfix really, and making the proposal match reality now.

1. 5a8ec8c revises the ownership semantics of Task initializers. They were previously documented to NOT own the task executors and pushed the responsibility onto developers to keep executors alive (similar to serial executors).
   • We can do better and have implemented initializers taking ownership of task executors passed to unstructured task initializers: [6.0][Concurrency] TaskExecutor ownership fixes  swift#74174

Removed some changes since not necessary anymore

2. 3ad8d51 is a necessary revision in order to uphold default actor isolation safety when a task executor is used. Without this change task executors could violate actor isolation and even run concurrent code on a single actor. We must address this by forwarding the serial executor to the task execution such that it can do the usual actor locking.
   • The implementation for this change is here: [Concurrency] Fix default actor isolation issue in face of TaskExecutor preference swift#74658 and we need to decide how aggressively we can phase out the incorrect APIs.
   • This was fixed without changes to the API 👍