fdo#57061: Use a new MIGRATION4 flag file for profile migration LO 3 -> 4

...to avoid missing migrations on Windows and Mac OS X, where existing "3" user
profiles got the "MIGRATED" flag file erroneously added when running LO 3
already, presumably due to 6b522673 "default
config location has changed, look in old config dir when migrating."

Change-Id: I8dd50a36450c167d47411834cc06a754affadf6d

desktop/source/migration/migration.cxx

@@ -172,7 +172,7 @@ static const char ITEM_DESCRIPTOR_LABEL[] = "Label";
 
 bool MigrationImpl::alreadyMigrated()
 {
-    rtl::OUString MIGRATION_STAMP_NAME("/MIGRATED");
+    rtl::OUString MIGRATION_STAMP_NAME("/MIGRATED4");
     rtl::OUString aStr = m_aInfo.userdata + MIGRATION_STAMP_NAME;
     File aFile(aStr);
     // create migration stamp, and/or check its existence