use upstream pegasus repository

no longer maintaining the IGWN mirror of this repository

https://git.ligo.org/computing/sccb/-/issues/1596#note_1076452

requested review from @duncanmmacleod

assigned to @adam-mercer

@duncanmmacleod Could you please take a look at this? If you agree I'll make similar changes for the other tags.

approved this merge request

mentioned in commit 8d04d413

merged