Standard / 19115-3 / Fix maintenance date type in 19139 conversion

[fxprunayre]

Maintenance date was using codeListValue 'update' instead of 'nextUpdate'

See https://github.com/geonetwork/core-geonetwork/blob/main/schemas/iso19115-3.2018/src/main/plugin/iso19115-3.2018/loc/eng/codelists.xml#L76

Checklist

• I have read the contribution guidelines
• Pull request provided for main branch, backports managed with label
• Good housekeeping of code, cleaning up comments, tests, and documentation
• Clean commit history broken into understandable chucks, avoiding big commits with hundreds of files, cautious of reformatting and whitespace changes
• Clean commit messages, longer verbose messages are encouraged
• API Changes are identified in commit messages
• Testing provided for features or enhancements using automatic tests
• User documentation provided for new features or enhancements in manual
• Build documentation provided for development instructions in README.md files
• Library management using pom.xml dependency management. Update build documentation with intended library use and library tutorials or documentation

Funded by EEA.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud