Use Pull Request grouping to reduce PR noise

In October 2022 scala-steward-org/scala-steward#2714 introduced the capability to group together
pull requests. If you want, you can have all updates go into one single PR, or you can also group by criteria, so that noisy releasers (like AWS SDK) are grouped into one PR, while everything else goes into another PR (these might be more significant updates that people would want to consider more urgently).

The behaviour (I _think_) appears to be that once a grouped PR is created, it's _not_ updated with new updates, so you have to merge it before you get another round of updates - but I don't think that's so bad. It certainly dramatically reduces update noise.

I've been using it on https://github.com/guardian/play-secret-rotation and I'd say it behaves well.

This department-wide config change was discussed at the P&E Server-side meetup on 1st February 2023, to positive assent (I just never got around to making the PR until now!).

https://docs.google.com/document/d/1wtZPwICuEeMQ8Ga7ErUIMZsWalnsI0DpnvnkNI3Qy5U/edit#heading=h.vjxmdl6a7ue3

https://github.com/scala-steward-org/scala-steward/blob/dc85945c95c69cb8dce3d4b5f862e41ecd692417/docs/repo-specific-configuration.md?plain=1#L37-L72

scala-steward.conf

@@ -1,3 +1,8 @@
+pullRequests.grouping = [
+  { name = "aws", "title" = "AWS dependency updates", "filter" = [{"group" = "software.amazon.awssdk"}, {"group" = "com.amazonaws"}] },
+  { name = "non_aws", "title" = "Non-AWS dependency updates", "filter" = [{"group" = "*"}] }
+]
+
 # Only include dependencies which automatically release updates as frequently
 # as daily, without those dependencies having meaningful security value. With our
 # current process, the daily update PRs generated by Scala Steward for those