[FLINK-30501] Update Flink build instruction to deprecate Java 8 instead of requiring Java 11

[lindong28]

What is the purpose of the change

Flink 1.15 and later versions require at least Java 11 to build from sources [1], whereas the pom.xml specifies the source/target is 1.8. This inconsistency confuses users.

As mentioned in the FLINK-25247 title, the goal of that ticket is to "Inform users about deprecation". It will be better to inform users that "Java 8 is deprecated" instead of saying "Fink requires at least Java 11 to build", so that users have the right information to make the right choice for themselves.

Also note that Flink community is regularly running flink-ml benchmark for both Java 8 and Java 11 [2], which suggests that we are practically ensuring Java 8 is supported.

If we decide to official drop Java 8 support, Flink community probably should give explicit notice regarding the deprecation period so that users can be prepared for this breaking change. We can follow the Kafka website doc [3] for example, which says "Java 8, Java 11, and Java 17 are supported. Note that Java 8 support has been deprecated since Apache Kafka 3.0 and will be removed in Apache Kafka 4.0".

[1] https://nightlies.apache.org/flink/flink-docs-release-1.15/docs/flinkdev/building/
[2] http://codespeed.dak8s.net:8000/timeline/?ben=mapSink.F27_UNBOUNDED&env=2
[3] https://kafka.apache.org/documentation/

Brief change log

Update Flink doc to mention that "Java 8 is deprecated" instead of saying "Flink requires at least Java 11 to build".

Verifying this change

N/A.

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): no
• 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? N/A

[flinkbot]

CI report:

• fd81ec2 Azure: CANCELED
• 9b6868d Azure: PENDING
• 6619e10 UNKNOWN

Bot commands

The @flinkbot bot supports the following commands:

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

[reswqa]

I have also seen some people have the same confusion after reading the documentation. Thanks @lindong28 for the update, I only left some minor comments, please take a look~

[lindong28]

Thank you @reswqa for the review. I have updated the PR as suggested. Could you take another look?

[reswqa]

Thanks @lindong28 for addressing my comments, LGTM!

[reswqa]

A small question: do we need to pick the changes to the master branch?

[lindong28]

@reswqa Yes, the PR for the master branch can be found at #21557. Can you also help review that PR? Thanks!

[lindong28]

@reswqa One moment. Let me update that PR based on the comments.