Spring Prevent execution of Antora jobs on forks

Prior to this commit, any fork that copied the main branch only experienced daily failures at 10am UTC due to the scheduled execution of the update-antora-ui-spring workflow. With this change, the workflow jobs are executed only when under the spring-projects GitHub organization.

The failures of my fork: https://github.com/scordio/spring-framework/actions/workflows/update-antora-ui-spring.yml

An alternative solution would be disabling the workflow on each affected fork.

Comment From: snicoll

Thanks @scordio - that looks like an oversight on our end.