[code-reflection] RFR: Update JDK version in opgen project

[Ana Maria Mihalceanu]

This PR fixes the JDK version in `opgen` project. 
As Babylon is based now on JDK 26, the project setup should be based on JDK 26.

In a terminal window, running `mvn clean verify` from `cr-examples/onnx/pom.xml` failed with  


[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.11.0:compile (default-compile) on project onnx-opgen: Compilation failure
[ERROR] invalid source release 25 with --enable-preview
[ERROR]   (preview language features are only supported for release 26)

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Commit messages:
 - Update jdk version in opgen project.

Changes: https://git.openjdk.org/babylon/pull/635/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=635&range=00
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/babylon/pull/635.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/635/head:pull/635

PR: https://git.openjdk.org/babylon/pull/635