Update TZUpdater README

Please update Timezone Updater Tool README page(https://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html) as per below instructions:

Section- Tool Updates (https://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html#updates)
Add the below sub-section:
- 2.3.0 Enhancements
    The 2.3.0 release now supports the vanguard tzdata format which was not supported since tzdata2018e.
    Below two are the related bug fixes:
    - JDK-8230554: TZUpdater support for "vanguard" format (JSR-310 part only)
    - JDK-8229818: Add workaround to TZUpdater for converting tzdata files from vanguard to rearguard (javazic part only)


Section: Known Issues (https://www.oracle.com/technetwork/java/javase/tzupdater-readme-136440.html#issues)
Remove the first sub-section completely:
"tzdata2018e and later

Since the 2018e tzdata release, the TZUpdater tool will need to use the rearguard format of tzdata provided by IANA. Further information can be found at:

    https://bugs.openjdk.java.net/browse/JDK-8195595
    https://bugs.openjdk.java.net/browse/JDK-8212684

The rearguard format of tzdata is provided for compatibility purposes. The tz-announce mails contain links to the relevant rearguard formats which should be used with the TZUpdater tool. Use the -l feature of the tool to specify which tzdata bundle should be used.

For example:

java -jar tzupdater.jar -l https://web.cs.ucla.edu/~eggert/tz/release/2019a/tzdata2019a-rearguard.tar.gz "