dotty phase 2: scalafix ExplicitNonNullaryApply (#28949)

* scalafix ExplicitNonNullaryApply prepare

+ Temporarily use com.sandinh:sbt-scalafix because scalacenter/scalafix#1098
+ Add ExplicitNonNullaryApply rule to .scalafix.conf
+ Manually fix a NonNullaryApply case in DeathWatchSpec that cause
  `fixall` fail because ExplicitNonNullaryApply rule incorrectly rewrite
  `context unbecome` to `context unbecome()` instead of `context.unbecome()`

* scalafix ExplicitNonNullaryApply

fix by enabling only ExplicitNonNullaryApply rule in .scalafix.conf then:
```
% sbt -Dakka.build.scalaVersion=2.13.1
> fixall
```

* scalafmtAll

* Revert to ch.epfl.scala:sbt-scalafix

Co-authored-by: Bùi Việt Thành <thanhbv@sandinh.net>

akka-cluster-metrics/src/test/scala/akka/cluster/metrics/EWMASpec.scala

@@ -82,7 +82,7 @@ class EWMASpec extends AkkaSpec(MetricsConfig.defaultEnabled) with MetricsCollec
         // wait a while between each message to give the metrics a chance to change
         Thread.sleep(100)
         usedMemory = usedMemory ++ Array.fill(1024)(ThreadLocalRandom.current.nextInt(127).toByte)
-        val changes = collector.sample.metrics.flatMap { latest =>
+        val changes = collector.sample().metrics.flatMap { latest =>
           streamingDataSet.get(latest.name) match {
             case None => Some(latest)
             case Some(previous) =>