Update TZUpdater README

Please update the following Timezone Updater sections under https://www.oracle.com/java/technologies/javase/tzupdater-readme.html

Section-

1) Tool Updates

- 2.3.0 Enhancements
    - JDK-8229818: Add workaround to TZUpdater for converting tzdata files from vanguard to rearguard (JDK 7u only)

This bug needs to be made public.

Summary:

TZUpdater tool usage for JDK 7u only has a workaround that involves translating vanguard to rearguard format before computation of javazic data sources.

A new TZUpdater property "tzupdater.forceUseOfVanguard" is introduced as a fallback option if the TZUpdater fails due to unforeseen circumstances.

The new TZUpdater property can be enabled by setting
"-Dtzupdater.forceUseOfVanguard=true".

2) Known Issues

Please add:

tzdata2020a

The following issue is known to exist when processing tzdata2020a:

Due to formatting changes in the IANA-maintained tzdata bundle, the TZUpdater tool fails to update to tzdata2020a for JDK 7u.

The issue is being tracked under :
https://bugs.openjdk.java.net/browse/JDK-8245950

Please follow the bug for a temporary workaround.


tzdata2016g section can be removed as it is very old.