SI-7958 Deprecate methods `future` and `promise` in the `scala.concurrent` package object

- The corresponding `apply` methods in the `Future` and `Promise` objects
  should be used instead.

- Adjusted tests to use non-deprecated versions

- Fixed doc comments not to use deprecated methods

- Added comment about planned removal in 2.13.0
Merge pull request #3111 from phaller/issue/7958

SI-7958 Deprecate methods `future` and `promise` in the `scala.concurren...