Remove ClusterStateSerializationTests (elastic#130877)

These tests do not seem valuable anymore, now that multi-project has
been merged some time ago. Additionally, creating objects for a cluster
state this way is tricky, as those objects might get new
features/fields, which would break serialization.

Closes elastic#130872

Author: nielsbauman

muted-tests.yml

@@ -403,9 +403,6 @@ tests:
 - class: org.elasticsearch.backwards.MixedClusterClientYamlTestSuiteIT
   method: test {p0=mtermvectors/10_basic/Tests catching other exceptions per item}
   issue: https://github.com/elastic/elasticsearch/issues/122414
-- class: org.elasticsearch.cluster.ClusterStateSerializationTests
-  method: testSerializationPreMultiProject
-  issue: https://github.com/elastic/elasticsearch/issues/130872
 - class: org.elasticsearch.search.SearchWithRejectionsIT
   method: testOpenContextsAfterRejections
   issue: https://github.com/elastic/elasticsearch/issues/130821

server/src/test/java/org/elasticsearch/cluster/ClusterStateSerializationTests.java

@@ -10,6 +10,7 @@
 package org.elasticsearch.cluster.metadata;
 
 import org.elasticsearch.TransportVersion;
+import org.elasticsearch.TransportVersions;
 import org.elasticsearch.cluster.ClusterModule;
 import org.elasticsearch.cluster.Diff;
 import org.elasticsearch.cluster.coordination.CoordinationMetadata;
@@ -45,6 +46,7 @@
 import org.elasticsearch.persistent.PersistentTasksService;
 import org.elasticsearch.test.AbstractChunkedSerializingTestCase;
 import org.elasticsearch.test.ESTestCase;
+import org.elasticsearch.test.TransportVersionUtils;
 import org.elasticsearch.test.index.IndexVersionUtils;
 import org.elasticsearch.test.rest.ObjectPath;
 import org.elasticsearch.threadpool.ThreadPool;
@@ -613,6 +615,17 @@ public void testMultiProjectSerialization() throws IOException {
         }
     }
 
+    public void testUnableToSerializeNonDefaultProjectBeforeMultiProject() {
+        final var projectId = randomUniqueProjectId();
+        Metadata metadata = Metadata.builder().put(ProjectMetadata.builder(projectId)).build();
+
+        try (BytesStreamOutput output = new BytesStreamOutput()) {
+            output.setTransportVersion(TransportVersionUtils.getPreviousVersion(TransportVersions.MULTI_PROJECT));
+            var e = assertThrows(UnsupportedOperationException.class, () -> metadata.writeTo(output));
+            assertEquals("There is 1 project, but it has id [" + projectId + "] rather than default", e.getMessage());
+        }
+    }
+
     public void testGetNonExistingProjectThrows() {
         final List<ProjectMetadata> projects = IntStream.range(0, between(1, 3))
             .mapToObj(i -> randomProject(ProjectId.fromId("p_" + i), between(0, 5)))