Downgrade tzdata-java to 2019c

The RPM package tzdata-java-2020a is not compatible with Java (at least 11.0.6, but should be for every version), due to the rename of America/Godthab to America/Nuuk.
This package is responsible to update the java timezone (in CLDR locale provider), thus you don't need to do any manual action or wait for a java update to get the new tzdata.

The issue has been identified by Java Developers when they have upgraded the data, see the mailing list, and has been fixed in the same commit than the upgrade one.

The fix is scheduled for 11.0.9 while 11.0.7 has just been released (end of April 2020).
While waiting for the JDK fix, we should downgrade our tzdata-java package to 2019c which is the one bundled in java 11.0.7 and works as expected.