docs: Update redis to follow the latest spec (#6907)

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## What do these changes do?

I changed some Redis code from the documentation to follow the latest
API. It is based on redis-py>=4.2.x on which aioredis has been merged
and started to be maintained.

## Are there changes in behavior for the user?

Nope!

## Related issue number

There is no issue related to this PR.

## Checklist

- [x] I think the code is well written
- [ ] Unit tests for the changes exist
- [x] Documentation reflects the changes
- [ ] If you provide code modification, please add yourself to
`CONTRIBUTORS.txt`
  * The format is &lt;Name&gt; &lt;Surname&gt;.
  * Please keep alphabetical order, the file is sorted by names.
- [x] Add a new news fragment into the `CHANGES` folder
  * name it `<issue_id>.<type>` for example (588.bugfix)
* if you don't have an `issue_id` change it to the pr id after creating
the pr
  * ensure type is one of the following:
    * `.feature`: Signifying a new feature.
    * `.bugfix`: Signifying a bug fix.
    * `.doc`: Signifying a documentation improvement.
    * `.removal`: Signifying a deprecation or removal of public API.
* `.misc`: A ticket has been closed, but it is not of interest to users.
* Make sure to use full sentences with correct case and punctuation, for
example: "Fix issue with non-ascii contents in doctest text files."

Thank you for reading and any feedback is always welcomed!

Co-authored-by: Sam Bull <aa6bs0@sambull.org>
(cherry picked from commit 0af182a)

CHANGES/6907.doc

@@ -0,0 +1 @@
+Updated Redis code examples from the document to follow the latest API.

docs/web_advanced.rst

@@ -892,20 +892,19 @@ background tasks could be registered as an :attr:`Application.on_startup`
 signal handler or :attr:`Application.cleanup_ctx` as shown in the example
 below::
 
-
-  async def listen_to_redis(app):
-      try:
-          sub = await aioredis.create_redis(('localhost', 6379))
-          ch, *_ = await sub.subscribe('news')
-          async for msg in ch.iter(encoding='utf-8'):
-              # Forward message to all connected websockets:
-              for ws in app[websockets]:
-                  ws.send_str('{}: {}'.format(ch.name, msg))
-      except asyncio.CancelledError:
-          pass
-      finally:
-          await sub.unsubscribe(ch.name)
-          await sub.quit()
+  async def listen_to_redis(app: web.Application):
+      client = redis.from_url("redis://localhost:6379")
+      channel = "news"
+      async with client.pubsub() as pubsub:
+          await pubsub.subscribe(channel)
+          while True:
+              try:
+                  msg = await pubsub.get_message(ignore_subscribe_messages=True)
+                  if msg is not None:
+                      for ws in app["websockets"]:
+                          await ws.send_str("{}: {}".format(channel, msg))
+              except asyncio.CancelledError:
+                  break
 
 
   async def background_tasks(app):