add release note #8733

doc/release-notes/8733-oai_dc-date.md

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+For exports and harvesting in `oai_dc` format, if "Production Date" is not set, "Publication Date" is now used instead. This change is reflected in the [Dataverse 4+ Metadata Crosswalk][] linked from the [Appendix][] of the User Guide.
+
+[Dataverse 4+ Metadata Crosswalk]: https://docs.google.com/spreadsheets/d/10Luzti7svVTVKTA-px27oq3RxCUM-QbiTkm8iMd5C54/edit#gid=1901625433&range=K7
+[Appendix]: https://guides.dataverse.org/en/latest/user/appendix.html