Fixing isMigrated bug, when migrations failed we incorrectly ignored it. (#37674) (#38042)

Prior to this change, when migrations failed (for example when a
.kibana_1 index already exists and no alias does) the awaitMigration
promise would resolve incorrectly. This allowed subsequent plugins,
including Spaces, to proceed using the SavedObjectsClient/Repository
to create documents. This commonly caused a .kibana index to be created,
which would then block the creation of a .kibana alias, and everything
would grind to a halt.

src/legacy/server/saved_objects/migrations/core/index_migrator.ts

@@ -53,7 +53,7 @@ export class IndexMigrator {
       pollInterval: context.pollInterval,
 
       async isMigrated() {
-        return requiresMigration(context);
+        return !(await requiresMigration(context));
       },
 
       async runMigration() {