[NOTICE] Experimental Java 17 support now available on master

[Chesnay Schepler <ch...@apache.org>]

The master branch now builds and runs with Java 17 out-of-the-box.

Notes:
- a nightly cron build was set up, but I expect it to fail because some 
tests only run in the branches and thus weren't covered by my testing. 
I'll try to stabilize things over the weekend.
- The Python Kafka connector is known to still have issues: (FLINK-32327 
<https://issues.apache.org/jira/browse/FLINK-32327>)
   - due to this the Python tests are disabled on CI in Java 17 builds
- In Java 17 builds Scala is being bumped to 2.12.15; don't try to load 
a savepoint from a Java 8/11 build.
- In addition to all tests that are being skipped on Java 11, HBase 2.2 
has now also made the list.

Huge shout-out to Mathias Pohl for reviewing my seemingly endless stream 
of PRs in the last weeks.

If you run into any issues, please report it in FLINK-15736 
<https://issues.apache.org/jira/browse/FLINK-15736>.