[server] Increase space for Phabricator backups

Summary: The Phabricator database is larger than I expected. Currently it takes up 46 MiB for one backup, so I'm increasing the space we have allocated for these backups.

Test Plan: #testinprod

Reviewers: palys-swm, KatPo

Reviewed By: KatPo

Subscribers: KatPo, zrebcu411, Adrian

Differential Revision: https://phabricator.ashoat.com/D467