[ci] Shrink platform-support Windows instances (#107912) (#107926)

(cherry picked from commit ed81cb5)

Author: brianseeders

.buildkite/pipelines/periodic-platform-support.yml

@@ -52,7 +52,7 @@ steps:
         agents:
           provider: gcp
           image: family/elasticsearch-{{matrix.image}}
-          machineType: n1-standard-32
+          machineType: n1-standard-16
           diskType: pd-ssd
           diskSizeGb: 350
         env: