fix: suppress warnings exposed by -Xfatal-warnings/-Werror

Remove unused classTag import in HttpMessage.scala.
Add -Wconf suppressions for structural type warning (Scala 2),
synchronized warning and override-deprecated warning (Scala 3).

Author: He-Pin

http-core/src/main/scala/org/apache/pekko/http/scaladsl/model/HttpMessage.scala

@@ -24,7 +24,7 @@ import scala.collection.immutable
 import scala.concurrent.{ ExecutionContext, Future }
 import scala.concurrent.duration._
 import scala.jdk.FutureConverters._
-import scala.reflect.{ classTag, ClassTag }
+import scala.reflect.ClassTag
 
 import org.apache.pekko
 import pekko.Done

project/Common.scala

@@ -41,9 +41,12 @@ object Common extends AutoPlugin {
       // Exhaustivity checking is only useful for simple sealed hierarchies and matches without filters.
       // In all other cases, the warning is non-actionable: you get spurious warnings that need to be suppressed
       // verbosely. So, opt out of those in general.
-      "-Wconf:cat=other-match-analysis&msg=match may not be exhaustive:s")).value,
+      "-Wconf:cat=other-match-analysis&msg=match may not be exhaustive:s",
+      "-Wconf:msg=inferred structural type:s")).value,
     scalacOptions ++= onlyOnScala3(Seq(
-      "-Werror")).value,
+      "-Werror",
+      "-Wconf:msg=Suspicious top-level unqualified call to synchronized:s",
+      "-Wconf:msg=overrides concrete, non-deprecated definition:s")).value,
     scalacOptions ++= onlyOnScala3Below39(Seq("-Yfuture-lazy-vals")).value,
     javacOptions ++=
       Seq("-encoding", "UTF-8", "--release", javacTarget),