[doc] Remove external herokuapp website link (elastic#77405)

The website is no longer live and is external to elastic domain. It was used as help for testing migration

# Conflicts:
#	docs/reference/migration/migrate_7_0/migrate_to_java_time.asciidoc

docs/reference/migration/migrate_7_0/migrate_to_java_time.asciidoc

@@ -231,9 +231,6 @@ In java time, `z` outputs 'Z' for Zulu when given a UTC timezone.
 We strongly recommend you test any date format changes using real data before
 deploying in production.
 
-For help with date debugging, consider using
-https://esddd.herokuapp.com/[https://esddd.herokuapp.com/.]
-
 [[java-time-migrate-update-mappings]]
 ==== Update index mappings
 To update joda-time date formats in index mappings, you must create a new index