[FLINK-36181] Use Java 17 by default

[MartijnVisser]

What is the purpose of the change

This PR changes Flink CI to run on Java 17 by default, effectively dropping support for Java 8. Since Java 11 is officially not deprecated, there's a separate Java profile to run JDK11 tests during the nightly cron builds.

Brief change log

• Changes CI to run on Java 17 by default, and removes Java 8 in the CI runs
• Bumps Scala to 2.12.20, since we're already in Flink 2.0 land and we can safely break this for the upgrade to Java 17.
• Updates README

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): don't know
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? docs

[flinkbot]

CI report:

• 7907aa1 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[snuyanzin]

@MartijnVisser I think you need to rebase to the latest master
the failure seems to be related to the bug in master fixed within FLINK-37006

[snuyanzin]

Thanks for addressing comments

LGTM in general

One thing would be nice to have: a test run of nightly jobs before merging (not sure whether it is possible now)

[snuyanzin]

Last minute comment
I noticed we have also scala version mentioned at

flink/flink-rpc/flink-rpc-akka/pom.xml

Line 42 in 105720f

<scala.version>2.12.16</scala.version>

how about sync it with the one in main pom?

[MartijnVisser]

@flinkbot run azure