[CI] Bump memory on PR part-4 agents (#120947)

(cherry picked from commit 10d09a1d717183c4364acb32787a63c89624d64c)

.buildkite/pipelines/pull-request/part-4-fips.yml

@@ -7,5 +7,5 @@ steps:
     agents:
       provider: gcp
       image: family/elasticsearch-ubuntu-2004
-      machineType: custom-32-98304
+      machineType: n1-standard-32
       buildDirectory: /dev/shm/bk

.buildkite/pipelines/pull-request/part-4.yml

@@ -7,5 +7,5 @@ steps:
     agents:
       provider: gcp
       image: family/elasticsearch-ubuntu-2004
-      machineType: custom-32-98304
+      machineType: n1-standard-32
       buildDirectory: /dev/shm/bk