RFR of CSR for start of JDK 20 updates JDK-8286094 and JDK-8286096

[Joseph D. Darcy]

Hello,

Please review several CSRs ahead of the future start-of-JDK-20 updates:

     JDK-8286094: Add source 20 and target 20 to javac
     https://bugs.openjdk.java.net/browse/JDK-8286094

     JDK-8286096: Add SourceVersion.RELEASE_20
     https://bugs.openjdk.java.net/browse/JDK-8286096

The code review of these changes will eventually be done under

     https://github.com/openjdk/jdk/pull/8236

I'm keeping that PR in draft for now to avoid spamming several mailing 
lists as the branch is synced with new changes from mainline.

Thanks,

-Joe