Fix discussion of extending-the-deleted-project-life

[pbchase]

Review text at https://ctsit.github.io/rcc.billing/articles/novel_concepts.html#extending-the-deleted-project-life. It is outdated because the feature provided by that patch is now part of REDCap Core. We should mention the addition of that feature and discuss how and why we use it.

This section needs to reflect the use of Number of days between project deletion and purging the project from the system on the General Settings page in Control Center