chore: downgrade TransitionUtils TTD log to INFO and clarify log message (besu-eth#9280)

Signed-off-by: Arnesh <arneshdadhich0011@gmail.com>
Co-authored-by: Arnesh Dadhich <arneshdadhich@Arneshs-MacBook-Pro.local>
Co-authored-by: Sally MacFarlane <macfarla.github@gmail.com>

Author: 3 people

consensus/merge/src/main/java/org/hyperledger/besu/consensus/merge/TransitionUtils.java

@@ -140,8 +140,9 @@ public static boolean isTerminalProofOfWorkBlock(
     }
 
     if (currentChainTotalDifficulty.isZero()) {
-      LOG.atWarn()
-          .setMessage("unable to get total difficulty for {}, parent hash {} difficulty not found")
+      LOG.atInfo()
+          .setMessage(
+              "Total difficulty unavailable for {} (parent: {}). Expected during transition from Clique/PoW to PoS or at merge-at-genesis.")
           .addArgument(header::toLogString)
           .addArgument(header::getParentHash)
           .log();