Remove milestone from MIMA checks

And ensure the ones to come aren't added either

project/Mima.scala

@@ -10,7 +10,6 @@ object Mima {
 
   // Important: the line with the "binary compatibility versions" comment below is matched during releases
   def binaryCompatibilityVersions = Set(
-    "1.1.0-M1",
     "" // binary compatibility versions
   )
 

project/Release.scala

@@ -244,6 +244,8 @@ object Release {
 
   val mimaVersionsPattern = s"(?m)^(\\s+)${Pattern.quote("\"\" // binary compatibility versions")}$$".r
 
+  val milestonePattern = ("[^-]+" + Pattern.quote("-M") + "[0-9]+(-[0-9]+)*").r
+
   val updateMimaVersions = ReleaseStep { state =>
 
     val vcs = state.vcs
@@ -253,28 +255,33 @@ object Release {
       sys.error(s"${ReleaseKeys.versions.label} key not set")
     }
 
-    val baseDir = Project.extract(state).get(baseDirectory.in(ThisBuild))
-    val mimaScalaFile = baseDir / "project" / "Mima.scala"
+    if (milestonePattern.unapplySeq(releaseVer).isEmpty) {
 
-    val content = Source.fromFile(mimaScalaFile)(Codec.UTF8).mkString
+      val baseDir = Project.extract(state).get(baseDirectory.in(ThisBuild))
+      val mimaScalaFile = baseDir / "project" / "Mima.scala"
 
-    mimaVersionsPattern.findAllIn(content).toVector match {
-      case Seq() => sys.error(s"Found no matches in $mimaScalaFile")
-      case Seq(_) =>
-      case _ => sys.error(s"Found too many matches in $mimaScalaFile")
-    }
+      val content = Source.fromFile(mimaScalaFile)(Codec.UTF8).mkString
 
-    val newContent = mimaVersionsPattern.replaceAllIn(
-      content,
-      m => {
-        val indent = m.group(1)
-        indent + "\"" + releaseVer + "\",\n" +
-          indent + "\"\" // binary compatibility versions"
+      mimaVersionsPattern.findAllIn(content).toVector match {
+        case Seq() => sys.error(s"Found no matches in $mimaScalaFile")
+        case Seq(_) =>
+        case _ => sys.error(s"Found too many matches in $mimaScalaFile")
       }
-    )
 
-    Files.write(mimaScalaFile.toPath, newContent.getBytes(StandardCharsets.UTF_8))
-    vcs.add(mimaScalaFile.getAbsolutePath).!!(log)
+      val newContent = mimaVersionsPattern.replaceAllIn(
+        content,
+        m => {
+          val indent = m.group(1)
+          indent + "\"" + releaseVer + "\",\n" +
+            indent + "\"\" // binary compatibility versions"
+        }
+      )
+
+      Files.write(mimaScalaFile.toPath, newContent.getBytes(StandardCharsets.UTF_8))
+      vcs.add(mimaScalaFile.getAbsolutePath).!!(log)
+    } else
+      state.log.info(s"$releaseVer is a milestone release, not adding it to the MIMA checks")
+
 
     state
   }