add support for building java21 code

[pjfanning]

#1023 added java 21 code to our docs. I don't believe that we have support for building this code to see if compiles. We have https://github.com/apache/incubator-pekko/blob/main/project/Jdk9.scala and can look into adding a Java 21 equivalent.

[He-Pin]

I can do that on weekend, I was planning to open issue like this.

And I think this is a good first issue too

Note: Community who interest this can chips in too.

[naosense]

@He-Pin hi, I want to take this, can you guide me on how to do it?

[He-Pin]

@naosense #1072 & #1054 Can you help check it really works, thanks.

[pjfanning]

I think we have made enough progress to close this.