ci: fix update of plugin or theme fields

See [1] for the reference of value field (ProjectV2FieldValue type),
and [2] for sample application code.

[1] https://docs.github.com/en/graphql/reference/input-objects#projectv2fieldvalue
[2] https://docs.github.com/en/enterprise-cloud@latest/issues/planning-and-tracking-with-projects/automating-your-project/automating-projects-using-actions#example-workflow-authenticating-with-a-github-app