Update copyright statement and license to 2023 W3C software and document license #577

In resolving #558 during issue triage, we realized that we might need to upgrade the license and copyright info to the latest version for 2023. W3C has become an independent entity and can hold the copyright directly. We'll need to check with W3C about update both Core and Vocabulary and whether this should be done immediately or wait for other changes to be published.