Update TZUpdater README

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

Section- System Requirements:

Please remove the existing System Requirements contents and add the below:

The TZUpdater tool supports all currently supported versions of the JDK. Prior to version 2.3.1, the tool only worked on Oracle binaries.

Section- Tool Updates:

Please add:

- 2.3.1 Enhancement/Bug Fix:

  - JDK-8245460: Allow all JDK vendors to work with TZUpdater

Removed restriction of letting TZUpdater update only Oracle JDK binaries. The TZUpdater tool now allows updates to any JDK binary.

  - JDK-8245950: TZUpdater cannot update to tzdata2020a for JDK 7u