Needed before jenkinsci/jenkins#6867 can get merged.

codemirror-config attribute is expected to be a valid JSON except the start and end bracket as per https://github.com/jenkinsci/jenkins/blob/cd3b1517e7a553fd6fcf39e25c4bd1e18fbe39de/core/src/main/resources/lib/form/textarea.jelly#L70-L73.

In the upstream change JSON#parse is used instead of eval to transform the codemirror-config into a JSON object. JSON#parse cannot parse function attributes, hence the drop of onBlur handler in this PR. There seems to be no need for it because this code re-adds it.

Testing done

Tested interactively against jenkinsci/jenkins#6867. Created a freestyle job, added the PowerShell build step. Made sure codemirror-config is parsed correctly, and no errors are thrown in the developer console.

### Submitter checklist
- [x] Make sure you are opening from a **topic/feature/bugfix branch** (right side) and not your main branch!
- [x] Ensure that the pull request title represents the desired changelog entry
- [x] Please describe what you did
- [x] Link to relevant issues in GitHub or Jira
- [x] Link to relevant pull requests, esp. upstream and downstream changes
- [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue