Add cleanup to list of potential labels for PRs (#3268)

I created a PR to cleanup some whitespace, and there was no good label
for it. I saw that we already had a
https://github.com/FoundationDB/fdb-record-layer/labels/cleanup label,
so adding that to the Build/Test/Documentation group in the release
notes seems like a good idea to me.

The description for
https://github.com/FoundationDB/fdb-record-layer/labels/cleanup is
`Stylistic code cleanup`, but I'm not sure if we should have a more
descriptive label, to avoid other types of cleanup, such as deleting
unused, private code.

Author: ScottDugas

build/release-notes-config.json

@@ -10,12 +10,13 @@
     "labels": ["performance"]},
    {"title": "Dependency Updates",
     "labels": ["dependencies"]},
-   {"title": "Build/Test/Documentation Improvements",
+   {"title": "Build/Test/Documentation/Style Improvements",
     "collapsed": true,
     "labels": [
       "build improvement",
       "documentation",
-      "testing improvement"]}
+      "testing improvement",
+      "cleanup"]}
  ],
  "catch_all": "Other Changes"
 }