Skip shard deprecation messages in REST tests (#30630)

A 6.x node can send a deprecation message that the default number of
shards will change from five to one in 7.0.0. In a mixed cluster,
whether or not a create index request sees five or one shard and
produces a deprecation message depends on the version of the master
node. This means that during BWC tests a test can see this deprecation
message depending on the version of the master node. In 6.x when we
introduced this deprecation message we assumed that whereever we see
this deprecation message is expected. However, in a mixed cluster test
we need a similar mechanism but it would only apply if the version of
the master node is earlier than 7.0.0. This commit takes advantage of a
recent change to expose the version of the master node to do sections of
REST tests. With this in hand, we can skip asserting on the deprecation
message if the version of the master node is before 7.0.0 and otherwise
seeing that deprecation message would be completely unexpected.

test/framework/src/main/java/org/elasticsearch/test/rest/yaml/section/DoSection.java

@@ -20,6 +20,7 @@
 package org.elasticsearch.test.rest.yaml.section;
 
 import org.apache.logging.log4j.Logger;
+import org.elasticsearch.Version;
 import org.elasticsearch.common.ParsingException;
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.collect.Tuple;
@@ -233,7 +234,7 @@ public class DoSection implements ExecutableSection {
                 }
                 fail(formatStatusCodeMessage(response, catchStatusCode));
             }
-            checkWarningHeaders(response.getWarningHeaders());
+            checkWarningHeaders(response.getWarningHeaders(), executionContext.masterVersion());
         } catch(ClientYamlTestResponseException e) {
             ClientYamlTestResponse restTestResponse = e.getRestTestResponse();
             if (!Strings.hasLength(catchParam)) {
@@ -259,7 +260,7 @@ public class DoSection implements ExecutableSection {
     /**
      * Check that the response contains only the warning headers that we expect.
      */
-    void checkWarningHeaders(final List<String> warningHeaders) {
+    void checkWarningHeaders(final List<String> warningHeaders, final Version masterVersion) {
         final List<String> unexpected = new ArrayList<>();
         final List<String> unmatched = new ArrayList<>();
         final List<String> missing = new ArrayList<>();
@@ -271,8 +272,19 @@ public class DoSection implements ExecutableSection {
             final boolean matches = matcher.matches();
             if (matches) {
                 final String message = matcher.group(1);
-                if (expected.remove(message) == false) {
-                    unexpected.add(header);
+                // noinspection StatementWithEmptyBody
+                if (masterVersion.before(Version.V_7_0_0_alpha1)
+                        && message.equals("the default number of shards will change from [5] to [1] in 7.0.0; "
+                        + "if you wish to continue using the default of [5] shards, "
+                        + "you must manage this on the create index request or with an index template")) {
+                    /*
+                     * This warning header will come back in the vast majority of our tests that create an index when running against an
+                     * older master. Rather than rewrite our tests to assert this warning header, we assume that it is expected.
+                     */
+                } else {
+                    if (expected.remove(message) == false) {
+                        unexpected.add(header);
+                    }
                 }
             } else {
                 unmatched.add(header);