Remove docs references to igwn-py38-testing
Before removing the igwn-py38-staging and -stable environments, this MR removes the docs reference on the master branch to igwn-py38-testing.
Maintenance will be performed on git.ligo.org, containers.ligo.org, and docs.ligo.org on Tuesday 7 Mayl 2024 starting at approximately 9am PDT. It is expected to take around twenty minutes and there will be several periods of downtime throughout the maintenance. Please address any comments, concerns, or questions to the helpdesk.
Before removing the igwn-py38-staging and -stable environments, this MR removes the docs reference on the master branch to igwn-py38-testing.