Update copyright year to 2021

[radazzouz]

Details

Update copyright year to 2021.

Acceptance Criteria

• When approved, right before merging, rebase with master and increment the package version in package.json and plugin.xml(see example commit: 09d5c6b).
• Create a new release (and tag) with the new package version (see https://github.com/PSPDFKit/PSPDFKit-Cordova/releases).
• Locally, pull the latest master and publish (npm publish) the new release to npm.

[steviki]

Looks good to me.

There are a few more places to update the year though, like the Read me and the license files. This is the commit from last year that touched a few more places: 0db78b9