[code-reflection] RFR: Update JDK version in opgen project [v3]

[Ana Maria Mihalceanu]

> This PR fixes the JDK version in `opgen` project. 
> It also removes a duplicate branch in a `switch` of `OnnxProtoBuilder.java`.
> 
> 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/opgen/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)

Ana Maria Mihalceanu has updated the pull request incrementally with one additional commit since the last revision:

  Fix switch compilation error after merge.
  
  Signed-off-by: ammbra <mihalceanuana at gmail.com>

-------------

Changes:
  - all: https://git.openjdk.org/babylon/pull/635/files
  - new: https://git.openjdk.org/babylon/pull/635/files/3c3e93fa..30c70097

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=babylon&pr=635&range=02
 - incr: https://webrevs.openjdk.org/?repo=babylon&pr=635&range=01-02

  Stats: 4 lines in 1 file changed: 0 ins; 4 del; 0 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