Rewrite asyncio subprocesses without child watchers

[kumaraditya303]

### Tasks
- [x] Remove child watchers (excludes threaded and pidfd watcher) #120805
- [x] Remove `get_child_watcher` and `set_child_watcher` https://github.com/python/cpython/pull/120818
- [x] Remove threaded and pidfd watcher https://github.com/python/cpython/pull/120893
- [x] Remove abc of it https://github.com/python/cpython/pull/120893
- [x] Add documentation regarding it and news entry https://github.com/python/cpython/pull/120895

Each task item will be done in a separate PR and news entry will be added when all of this is done otherwise it will be confusing for users.

Linked PRs

• GH-120804: Remove SafeChildWatcher, FastChildWatcher and MultiLoopChildWatcher from asyncio #120805
• GH-120804: Remove get_child_watcher and set_child_watcher from asyncio #120818
• GH-120804: Remove PidfdChildWatcher, ThreadedChildWatcher and AbstractChildWatcher from asyncio APIs #120893
• GH-120804: add docs for removal for asyncio child watchers #120895
• gh-120804: remove is_active method from internal child watchers implementation in asyncio #121124