Java V2 updates the POM files to use JDK 21

[scmacdon]

This pull request updates the POM files to use JDK 21

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[tkhill-AWS]

It looks like there are still a couple of 17 in the bedrock examples:

javav2/example_code/bedrock
pom.xml
        <java.version>17</java.version>

javav2/example_code/bedrock-agents-runtime
pom.xml
    <java.version>17</java.version>

[tkhill-AWS]

Run the following prompt in ChatGPT:

In the context of maven, do I need to specify both properties 'java.version' and 'maven.compiler.target' together with 'maven.compiler.source'?

We can do away with maven.compiler.target and maven.compiler.source

[tkhill-AWS]

I put a couple comments in the main PR page.