[TEST] Fix more side effects of elastic/x-pack-elasticsearch#2975 on machines with < 16GB RAM

Our rolling upgrade tests were failing on many machines due to using
the 5.x default of 4GB model_memory_limit, which then propagated forward
to 6.1+ even though the default is now lower.

Original commit: elastic/x-pack-elasticsearch@3b23d8fe9d

qa/rolling-upgrade/src/test/resources/rest-api-spec/test/mixed_cluster/30_ml_jobs_crud.yml

@@ -62,6 +62,9 @@ setup:
                 "bucket_span": "60s",
                 "detectors" :[{"function":"metric","field_name":"responsetime","by_field_name":"airline"}]
             },
+            "analysis_limits" : {
+                "model_memory_limit": "50mb"
+            },
             "data_description" : {
                 "format":"xcontent",
                 "time_field":"time",

qa/rolling-upgrade/src/test/resources/rest-api-spec/test/mixed_cluster/40_ml_datafeed_crud.yml

@@ -40,6 +40,9 @@ setup:
                 "bucket_span": "60s",
                 "detectors" :[{"function":"count"}]
             },
+            "analysis_limits" : {
+                "model_memory_limit": "50mb"
+            },
             "data_description" : {
                 "format":"xcontent",
                 "time_field":"time"

qa/rolling-upgrade/src/test/resources/rest-api-spec/test/old_cluster/30_ml_jobs_crud.yml

@@ -14,6 +14,9 @@
                 "bucket_span": "60s",
                 "detectors" :[{"function":"metric","field_name":"responsetime","by_field_name":"airline"}]
             },
+            "analysis_limits" : {
+                "model_memory_limit": "50mb"
+            },
             "data_description" : {
                 "format":"xcontent",
                 "time_field":"time",
@@ -62,6 +65,9 @@
                 "bucket_span": "60s",
                 "detectors" :[{"function":"count","field_name":""}]
             },
+            "analysis_limits" : {
+                "model_memory_limit": "50mb"
+            },
             "data_description" : {
                 "format":"xcontent",
                 "time_field":"time",

qa/rolling-upgrade/src/test/resources/rest-api-spec/test/old_cluster/40_ml_datafeed_crud.yml

@@ -14,6 +14,9 @@
                 "bucket_span": "60s",
                 "detectors" :[{"function":"count"}]
             },
+            "analysis_limits" : {
+                "model_memory_limit": "50mb"
+            },
             "data_description" : {
                 "format":"xcontent",
                 "time_field":"time"