Update minimum transport version in main

Versions 9.1.0 onwards will not be wire-compatible with versions before
8.19.0. This commit sets the minimum transport version to reject
handshakes from earlier versions.

Author: DaveCTurner

server/src/main/java/org/elasticsearch/TransportVersions.java

@@ -263,7 +263,7 @@ static TransportVersion def(int id) {
      * Reference to the earliest compatible transport version to this version of the codebase.
      * This should be the transport version used by the highest minor version of the previous major.
      */
-    public static final TransportVersion MINIMUM_COMPATIBLE = BYTE_SIZE_VALUE_ALWAYS_USES_BYTES_1;
+    public static final TransportVersion MINIMUM_COMPATIBLE = INITIAL_ELASTICSEARCH_8_19;
 
     /**
      * Reference to the minimum transport version that can be used with CCS.