Deprecate externalPom

Ref #5493

core-macros/src/main/scala/sbt/internal/util/appmacro/ContextUtil.scala

@@ -157,7 +157,7 @@ final class ContextUtil[C <: blackbox.Context](val ctx: C) {
     case _ => ()
   }
 
-  @deprecated("1.3.0", "Use that variant that specifies the M instance types to exclude")
+  @deprecated("Use that variant that specifies the M instance types to exclude", since = "1.3.0")
   /**
    * A function that checks the provided tree for illegal references to M instances defined in the
    *  expression passed to the macro and for illegal dereferencing of M instances.

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

@@ -4279,6 +4279,10 @@ trait BuildExtra extends BuildCommon with DefExtra {
     baseDirectory.value / name
   }
 
+  @deprecated(
+    "externalIvyFile is not supported by Couriser, and will be removed in the future",
+    since = "1.5.0"
+  )
   def externalIvyFile(
       file: Initialize[File] = inBase("ivy.xml"),
       iScala: Initialize[Option[ScalaModuleInfo]] = scalaModuleInfo
@@ -4290,6 +4294,10 @@ trait BuildExtra extends BuildCommon with DefExtra {
       managedScalaInstance.value
     )
 
+  @deprecated(
+    "externalPom is not supported by Coursier, and will be removed in the future",
+    since = "1.5.0"
+  )
   def externalPom(
       file: Initialize[File] = inBase("pom.xml"),
       iScala: Initialize[Option[ScalaModuleInfo]] = scalaModuleInfo,