[FLINK-33159] [build] use variables in pom file for Java and Maven versions

[davidradl]

What is the purpose of the change

Use Maven variables for all cases when referring to Java and Maven versions in the pom file

Brief change log

I have introduced 2 Maven variables for the 2 versions that are referenced in the build.
I have ensured all references to Java 1.8 use the java variable

Verifying this change

Please make sure both new and modified tests in this PR follows the conventions defined in our code quality guide: https://flink.apache.org/contributing/code-style-and-quality-common.html#testing

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

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

-no

Documentation

• Does this pull request introduce a new feature? ( no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

[flinkbot]

CI report:

• 3ca812e Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

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

[davidradl]

thanks @MartijnVisser . I am looking forward to your review @zentol :-) . If we are ok with this, please could you assign me the issue and pr.

[ferenc-csaky]

Thank you for your contribution! Added 2 comments. In general I believe if we change these props it would worth to explore the whole codebase, so we do not leave any loose ends.

After a quick check I found that the POM of flink-walkthrough-datastream-java archetype defines a raw 1.8 as target.java.version, but it should probably inherit that value from the root POM variable as well with the @target.java.version@ idiom.

[davidradl]

@ferenc-csaky

In general I believe if we change these props it would worth to explore the whole codebase, so we do not leave any loose ends.
I could not see any other places in this repository we nee to change

[ferenc-csaky]

Added 1 additional comment, the rest seems right.

[davidradl]

@ferenc-csaky thanks I have made the requested change.

[ferenc-csaky]

Thanks for the improvements, LGTM!

[davidradl]

@zentol are you ok to merge this please?

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