Retrofit json-java21-jtd to download test data from upstream

[simbo1905]

The 'json-java21-jdt' module now downloads its test suite from the upstream Microsoft repository using 'download-maven-plugin', keeping the repo clean of large test artifacts.

We should apply the same pattern to 'json-java21-jtd', which currently commits 'src/test/resources/jtd-test-suite.zip'.

Tasks:

1. Update 'json-java21-jtd/pom.xml' to use 'download-maven-plugin'.
2. Update tests to look for data in 'target/test-data'.
3. Remove 'src/test/resources/jtd-test-suite.zip'.