SOLR-16891: Fix blank Cloud graph screen in UI (apache#1783)

* MapWriter and IteratorWriter should not be special cases for JSON writing
* DocCollection, Slice and Replica now write to Json and write to Maps identically.

(cherry picked from commit 952a7be)

solr/CHANGES.txt

@@ -300,6 +300,8 @@ Bug Fixes
 
 * SOLR-16753: PRS state is now always updated at the end of a splitShard. (Houston Putman, hossman)
 
+* SOLR-16891: DocCollection, Slice and Replica now write to Json and write to Maps identically. (Houston Putman, Tomás Fernández Löbbe)
+
 Dependency Upgrades
 ---------------------
 * PR#1744: Update software.amazon.awssdk:* to v2.20.97 (solrbot)

solr/solrj/src/java/org/apache/solr/common/util/Utils.java

@@ -229,7 +229,7 @@ public static Number intIfNotOverflown(long n) {
   public static byte[] toJSON(Object o) {
     if (o == null) return new byte[0];
     CharArr out = new CharArr();
-    new JSONWriter(out, 0).write(o); // indentation by default
+    new JSONWriter(out, 2).write(o); // indentation by default
     return toUTF8(out);
   }