feat: Use _sbt2_3 suffix

**Problem**
The extra attribute is a vestige from the days when sbt plugins were published on Ivy repos.

**Solution**
This uses normal suffix system.

main/src/main/scala/sbt/Defaults.scala

@@ -2725,10 +2725,17 @@ object Defaults extends BuildCommon {
     }
 
   def sbtPluginExtra(m: ModuleID, sbtV: String, scalaV: String): ModuleID =
-    m.extra(
-      PomExtraDependencyAttributes.SbtVersionKey -> sbtV,
-      PomExtraDependencyAttributes.ScalaVersionKey -> scalaV
-    ).withCrossVersion(Disabled())
+    partialVersion(sbtV) match
+      case Some((0, _)) | Some((1, _)) =>
+        m.extra(
+          PomExtraDependencyAttributes.SbtVersionKey -> sbtV,
+          PomExtraDependencyAttributes.ScalaVersionKey -> scalaV
+        ).withCrossVersion(Disabled())
+      case Some(_) =>
+        // this produces a normal suffix like _sjs1_2.13
+        val prefix = s"sbt${binarySbtVersion(sbtV)}_"
+        m.cross(CrossVersion.binaryWith(prefix, ""))
+      case None => sys.error(s"unknown sbt version $sbtV")
 
   def discoverSbtPluginNames: Initialize[Task[PluginDiscovery.DiscoveredNames]] =
     (Def.task { sbtPlugin.value }).flatMapTask { case p =>
@@ -3073,7 +3080,7 @@ object Classpaths {
     Defaults.globalDefaults(
       Seq(
         publishMavenStyle :== true,
-        sbtPluginPublishLegacyMavenStyle := true,
+        sbtPluginPublishLegacyMavenStyle :== true,
         publishArtifact :== true,
         (Test / publishArtifact) :== false
       )
@@ -3081,8 +3088,10 @@ object Classpaths {
 
   private lazy val publishSbtPluginMavenStyle = Def.task(sbtPlugin.value && publishMavenStyle.value)
   private lazy val packagedDefaultArtifacts = packaged(defaultArtifactTasks)
-  private lazy val emptyArtifacts = Def.task(Map.empty[Artifact, HashedVirtualFileRef])
-
+  private lazy val sbt2Plus: Def.Initialize[Boolean] = Def.setting {
+    val sbtV = (pluginCrossBuild / sbtBinaryVersion).value
+    sbtV != "1.0" && !sbtV.startsWith("0.")
+  }
   val jvmPublishSettings: Seq[Setting[_]] = Seq(
     artifacts := artifactDefs(defaultArtifactTasks).value,
     packagedArtifacts := Def
@@ -3102,13 +3111,39 @@ object Classpaths {
    * valid POM file, that is a POM file that Maven can resolve.
    */
   private def mavenArtifactsOfSbtPlugin: Def.Initialize[Task[Map[Artifact, HashedVirtualFileRef]]] =
+    Def.task {
+      // This is a conditional task. The top-level must be an if expression.
+      if (sbt2Plus.value) {
+        // Both POMs and JARs are Maven-compatible in sbt 2.x, so ignore the workarounds
+        packagedDefaultArtifacts.value
+      } else {
+        val crossVersion = sbtCrossVersion.value
+        val legacyPomArtifact = (makePom / artifact).value
+        val converter = fileConverter.value
+        def addSuffix(a: Artifact): Artifact = a.withName(crossVersion(a.name))
+        Map(
+          addSuffix(legacyPomArtifact) -> converter.toVirtualFile(
+            makeMavenPomOfSbtPlugin.value.toPath()
+          )
+        ) ++
+          pomConsistentArtifactsForLegacySbt.value ++
+          legacyPackagedArtifacts.value
+      }
+    }
+
+  private def legacyPackagedArtifacts: Def.Initialize[Task[Map[Artifact, HashedVirtualFileRef]]] =
+    Def.task {
+      // This is a conditional task. The top-level must be an if expression.
+      if (sbtPluginPublishLegacyMavenStyle.value) packagedDefaultArtifacts.value
+      else Map.empty[Artifact, HashedVirtualFileRef]
+    }
+
+  private def pomConsistentArtifactsForLegacySbt
+      : Def.Initialize[Task[Map[Artifact, HashedVirtualFileRef]]] =
     Def.task {
       val crossVersion = sbtCrossVersion.value
-      val legacyArtifact = (makePom / artifact).value
-      val converter = fileConverter.value
-      val pom = converter.toVirtualFile(makeMavenPomOfSbtPlugin.value.toPath)
       val legacyPackages = packaged(defaultPackages).value
-      def addSuffix(a: Artifact): Artifact = a.withName(crossVersion(a.name))
+      val converter = fileConverter.value
       def copyArtifact(
           artifact: Artifact,
           fileRef: HashedVirtualFileRef
@@ -3120,11 +3155,9 @@ object Classpaths {
         IO.copyFile(file, targetFile)
         artifact.withName(nameWithSuffix) -> converter.toVirtualFile(targetFile.toPath)
       }
-      val packages = legacyPackages.map { case (artifact, file) => copyArtifact(artifact, file) }
-      val legacyPackagedArtifacts = Def
-        .ifS(sbtPluginPublishLegacyMavenStyle.toTask)(packagedDefaultArtifacts)(emptyArtifacts)
-        .value
-      packages + (addSuffix(legacyArtifact) -> pom) ++ legacyPackagedArtifacts
+      legacyPackages.map { case (artifact, file) =>
+        copyArtifact(artifact, file);
+      }
     }
 
   private def sbtCrossVersion: Def.Initialize[String => String] = Def.setting {