MINOR: [Dev] Add triage users (#14823)

Similarly as #14719

@ milesgranger has been contributing regularly the last few months both in PRs (https://github.com/apache/arrow/commits?author=milesgranger) as issue triage. Adding him to the collaborators (triage role) enables him to do that on github as well (disclaimer: Miles is a colleague of mine).

Authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>

Author: jorisvandenbossche

.asf.yaml

@@ -20,6 +20,7 @@ github:
   homepage: https://arrow.apache.org/
   collaborators:
     - assignUser
+    - milesgranger
     - raulcd
     - toddfarmer