Add Lucene 9.12.1 index version constant (elastic#118557)

This commit adds the IndexVersion constant for Lucene 9.12.1. Since main is already on Lucene 10, this change trivially just ensures that the constant version value in main is the same as that in the 8.x branch.

server/src/main/java/org/elasticsearch/index/IndexVersions.java

@@ -133,6 +133,7 @@ private static Version parseUnchecked(String version) {
     public static final IndexVersion TIME_BASED_K_ORDERED_DOC_ID_BACKPORT = def(8_520_00_0, Version.LUCENE_9_12_0);
     public static final IndexVersion V8_DEPRECATE_SOURCE_MODE_MAPPER = def(8_521_00_0, Version.LUCENE_9_12_0);
     public static final IndexVersion USE_SYNTHETIC_SOURCE_FOR_RECOVERY_BACKPORT = def(8_522_00_0, Version.LUCENE_9_12_0);
+    public static final IndexVersion UPGRADE_TO_LUCENE_9_12_1 = def(8_523_00_0, parseUnchecked("9.12.1"));
     public static final IndexVersion UPGRADE_TO_LUCENE_10_0_0 = def(9_000_00_0, Version.LUCENE_10_0_0);
     public static final IndexVersion LOGSDB_DEFAULT_IGNORE_DYNAMIC_BEYOND_LIMIT = def(9_001_00_0, Version.LUCENE_10_0_0);
     public static final IndexVersion TIME_BASED_K_ORDERED_DOC_ID = def(9_002_00_0, Version.LUCENE_10_0_0);