Revert mempool fee estimation back to "medium" priority

Reverts bisq-network#4585, which increased Bisq to always use the
"high" priority fee estimation. Now that we've improved mempool.space
"medium" fee estimates in mempool/mempool@15bb5a96 and addressed the
empty mempool issue as well, we can move Bisq back to using "medium",
since always using "high" would be a bit wasteful at times.

pricenode/src/main/java/bisq/price/mining/providers/MempoolFeeRateProvider.java

@@ -91,7 +91,7 @@ protected FeeRate doGet() {
 
     private long getEstimatedFeeRate() {
         return getFeeRatePredictions()
-            .filter(p -> p.getKey().equalsIgnoreCase("fastestFee"))
+            .filter(p -> p.getKey().equalsIgnoreCase("halfHourFee"))
             .map(Map.Entry::getValue)
             .findFirst()
             .map(r -> {