Updates SDK to Java21

[amwakefield-olson]

Update postmark-java SDK to Java 21 (v2.0.0)

Summary

This PR updates the postmark-java SDK to require Java 21 as the minimum runtime. It resolves a compression issue affecting Java 21 applications during HTTP response body conversion and modernizes the dependency stack.

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Changes

Core Java Update

• Set minimum Java version to Java 21 in pom.xml (java.version, compiler source/target)
• Marks this as a breaking change — Java 8/11 are no longer supported

Dependency Updates (pom.xml)

Dependency	Previous	Updated
httpclient5	—	5.5
jackson-core/annotations/databind	—	2.15.0
tika-core	—	3.2.3
mockito-core	—	4.110

CI Configuration (.circleci/config.yml)

• Added test matrix covering Java 21, 22, 23, 24, and 25 using cimg/openjdk
• Added Snyk security scan job with dependency monitoring (snyk monitor) scoped to exclude test artifacts

Changelog

• Added 2.0.0 entry documenting the Java 21 migration and compression fix

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Breaking Changes

• Java 21+ required — applications running on older JVM versions must upgrade before adopting this release

Testing

• All unit tests pass across the full Java 21–25 matrix via CircleCI

[amwakefield-olson]

Hello @ac-bgelfius!

Thanks for entertaining my PR. I was able to pull this into my own implementation of Postmark calls and validated it worked in the integration tests and ensured all unit tests worked as expected.

Please let me know if you have any questions.

Thanks,

Allan