Backport "Add versionLine marker to pom.xml" to LTS (#21008)

Backports #19986 to the LTS branch.

PR submitted by the release tooling.

Author: WojciechMazur

Diff for: project/Build.scala

@@ -85,6 +85,9 @@ object Build {
 
   val baseVersion = "3.3.4-RC1"
 
+  // LTS or Next
+  val versionLine = "LTS"
+
   // Versions used by the vscode extension to create a new project
   // This should be the latest published releases.
   // TODO: Have the vscode extension fetch these numbers from the Internet
@@ -1815,6 +1818,10 @@ object Build {
         "scm:git:[email protected]:scala/scala3.git"
       )
     ),
+    pomExtra :=
+      <properties>
+        <scala.versionLine>{versionLine}</scala.versionLine>
+      </properties>,
     developers := List(
       Developer(
         id = "odersky",