[Doc] migration guide joda (#51986)

The joda to java.time migration requires users to upgrade their mappings. We allow them to still use 6.x created indices with joda patterns in 7 but ask them to upgrade their patterns in 7.x.
This migration guide is to help them understand how they could be affected and what needs to be changed in their mappings.
closes #51614
closes #51236

Author: pgomulka

docs/reference/migration/migrate_7_0/java_time.asciidoc

@@ -89,6 +89,7 @@ When you are running Elasticsearch 7 with Java 8, you are not able to parse
 the timezone `GMT0` properly anymore. The reason for this is a bug in the
 JDK, which has not been fixed for JDK8. You can read more in the
 https://bugs.openjdk.java.net/browse/JDK-8138664[official issue]
+This bug is fixed in JDK9 and later versions.
 
 [float]
 ==== Scripting with dates should use java time based methods
@@ -125,3 +126,10 @@ and should be replaced.
 
 With the switch to java time, support for negative timestamps has been removed.
 For dates before 1970, use a date format containing a year.
+
+
+[float]
+==== Migration guide
+For a detailed migration guide, see <<migrate-to-java-time>>.
+
+include::migrate_to_java_time.asciidoc[]