Bumping gaia.json for 1 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/621040ee199c
Author: Ryan VanderMeulen <ryanvm@gmail.com>
Desc: Revert "Merge pull request #15356 from borjasalguero/dm_update_time_strings"

This reverts commit 78dda57ce4b5e5eb862a370a4fb994d1f0b1c5a3, reversing
changes made to 7c033555f5f79bca85d2a89a9d78fa3dff7dfead.
Bumping manifests a=b2g-bump