Pull snapshots prior to 6.3 from elastic maven (elastic/x-pack-elasticsearch#3900)

Snapshot versions before 6.3 do not exist in the public, so they need to
be pulled from snapshots.elastic.co/maven.

Original commit: elastic/x-pack-elasticsearch@3cc94add91

build.gradle

@@ -77,7 +77,7 @@ subprojects {
 
   bwcVersions.snapshotProjectNames.each { snapshotName ->
     Version snapshot = bwcVersions.getSnapshotForProject(snapshotName)
-    if (snapshot != null ) {
+    if (snapshot != null && snapshot.onOrAfter("6.3.0")) {
       String snapshotProject = ":x-pack-elasticsearch:plugin:bwc:${snapshotName}"
       project(snapshotProject).ext.bwcVersion = snapshot
         ext.projectSubstitutions["org.elasticsearch.plugin:x-pack:${snapshot}"] = snapshotProject

qa/full-cluster-restart/build.gradle

@@ -274,5 +274,8 @@ subprojects {
     maven {
       url "https://artifacts.elastic.co/maven"
     }
+    maven {
+      url "https://snapshots.elastic.co/maven"
+    }
   }
 }

qa/rolling-upgrade/build.gradle

@@ -293,5 +293,8 @@ subprojects {
     maven {
       url "https://artifacts.elastic.co/maven"
     }
+    maven {
+      url "https://snapshots.elastic.co/maven"
+    }
   }
 }