Adjust BWC version on OS pretty name

This commit adjusts the BWC version the OS pretty name field on OsInfo
now that this field has been backported to the 6.x development branch.

server/src/main/java/org/elasticsearch/monitor/os/OsInfo.java

@@ -61,7 +61,7 @@ public OsInfo(StreamInput in) throws IOException {
         this.availableProcessors = in.readInt();
         this.allocatedProcessors = in.readInt();
         this.name = in.readOptionalString();
-        if (in.getVersion().onOrAfter(Version.V_7_0_0)) {
+        if (in.getVersion().onOrAfter(Version.V_6_6_0)) {
             this.prettyName = in.readOptionalString();
         } else {
             this.prettyName = null;
@@ -76,7 +76,7 @@ public void writeTo(StreamOutput out) throws IOException {
         out.writeInt(availableProcessors);
         out.writeInt(allocatedProcessors);
         out.writeOptionalString(name);
-        if (out.getVersion().onOrAfter(Version.V_7_0_0)) {
+        if (out.getVersion().onOrAfter(Version.V_6_6_0)) {
             out.writeOptionalString(prettyName);
         }
         out.writeOptionalString(arch);