[doc] Improve joda-time migration guide (#60499)

Previously migration guide incorrectly stated that joda-time patterns have to be fixed before upgrading to 7.x
since (7.7) #52555 and our bwc policy 6.x created indices even with joda-time are supported
relates #60374

docs/reference/migration/migrate_7_0/migrate_to_java_time.asciidoc

@@ -5,16 +5,23 @@ With 7.0, {es} switched from joda time to java time for date-related parsing,
 formatting, and calculations. This guide is designed to help you determine
 if your cluster is impacted and, if so, prepare for the upgrade.
 
+You do not need to convert joda-time date formats to java time for indices
+created in {es} 6.8 before upgrading to 7.7 or later versions. However, mappings
+for indices created in 7.7 and later versions must use java-time formats.
+
+To ensure new indices use java-time formats, we recommend you update any ingest
+pipelines and index templates created in 6.8 to java time before upgrading. See:
+
+* <<java-time-migration-update-ingest-pipelines>>
+* <<java-time-migration-update-index-templates>>
+
+Indices created in versions 7.0-7.6 cannot use joda time. This was fixed
+was in 7.7 with {es-pull}52555[#52555].
 [discrete]
 [[java-time-convert-date-formats]]
 === Convert date formats
 
-To upgrade to 7.0, you'll need to convert any joda-time date formats
-to their java-time equivalents.
-
-To help track this effort, you can prefix java-time date formats with an `8`
- in {es} 6.8 and later versions.
-
+To use java time in 6.8, prefix the date format with an `8`.
 For example, you can change the date format `YYYY-MM-dd` to `8yyyy-MM-dd` to
 indicate the date format uses java time.
 
@@ -51,7 +58,7 @@ or the {kibana-ref}/upgrade-assistant.html[Kibana upgrade assistant].
 [[java-time-migration-incompatible-date-formats]]
 ==== Incompatible date formats
 Custom date formats containing the following joda-time literals should be
-changed before upgrading.
+converted to their java-time equivalents.
 
 `Y` (Year of era)::
 +
@@ -94,7 +101,7 @@ There is no replacement. Instead, we recommend you preprocess your input.
 --
 Replace with `Y`.
 
-In java time, `x` means https://docs.oracle.com/javase/8/docs/api/java/time/format/DateTimeFormatter.html[zone-offset]. 
+In java time, `x` means https://docs.oracle.com/javase/8/docs/api/java/time/format/DateTimeFormatter.html[zone-offset].
 
 [WARNING]
 ====
@@ -174,7 +181,7 @@ yyyy-MM-dd||yyyy-MM-d
 
 In java time, `d` also does not accept more than 2 digits. To accept days with more
 than two digits, you must include a text literal in your java-time date format.
-For example, to parse `2010-01-00001`, you must use the following java-time date format: 
+For example, to parse `2010-01-00001`, you must use the following java-time date format:
 
 [source,txt]
 --------------------------------------------------
@@ -192,7 +199,7 @@ For example, the joda-time date format `EEE YYYY-MM` accepts both
 `Wed 2020-01` and `Wednesday 2020-01`.
 
 To accept both of these dates in java time, you must specify each format using
-the `||` delimiter: 
+the `||` delimiter:
 
 [source,txt]
 --------------------------------------------------
@@ -278,8 +285,8 @@ GET my-index-000001/_mapping
 To change the date format for the `datetime` field, create a separate index
 containing an updated mapping and date format.
 
-For example, the following `my-index-000002` index changes the `datetime` field's 
-date format to `8uuuu/MM/dd HH:mm:ss||uuuu/MM/dd||epoch_millis`. The `8` prefix 
+For example, the following `my-index-000002` index changes the `datetime` field's
+date format to `8uuuu/MM/dd HH:mm:ss||uuuu/MM/dd||epoch_millis`. The `8` prefix
 indicates this date format uses java time.
 
 [source,console]