PP-7609 - Replace lambda with method references

Description:
- This PR replaces lambdas with method references
- With @alexbishop1

Author: nimalank7

src/main/java/uk/gov/pay/ledger/event/model/EventDigest.java

@@ -51,7 +51,8 @@ public static EventDigest fromEventList(List<Event> events) {
                 .orElseThrow(() -> new EmptyEventsException("No events found"));
 
         var latestSalientEventType = events.stream()
-                .map(e -> SalientEventType.from(e.getEventType()))
+                .map(Event::getEventType)
+                .map(SalientEventType::from)
                 .flatMap(Optional::stream)
                 .findFirst()
                 .orElse(null);