[6.0][Concurrency] Remove last usages of @_unsafeInheritExecutor

[ktoso]

Description: We still had a few stray uses of @_unsafeInheritExecutor which we are intending to stop using entirely. The uses were in task cancellation handler (which I noticed), and withTaskGroup*.
Scope/Impact: Use the official way of inheriting isolation, rather than the fragile unofficial attribute. Impact on users is more correct inheritance of context, making APIs usable in actors in Swift 6 mode.
Risk: Low, Verified ABI method remain and only adds #isolation new API
Testing: CI testing, added tests to verify behavior of task cancellation handler inside actor
Reviewed by: @hborla

Original PR: #73568
Radar: rdar://127874129

[ktoso]

@swift-ci please test

[DougGregor]

Looks great, thank you!

[ktoso]

Hmmm, there's one problem we need to discuss and solve about isolation of the closure still.
I.e. how should we properly handle:

actor A {
  func g() { }
  var num: Int = 0

  func f() async throws {
    await withTaskGroup(...) { group in
      g()
      num += 1
    }
  }
}

Blocking on rdar://125078448