Bumping gaia.json for 2 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/7912bfa2058e
Author: Arthur Chen <crh0716@gmail.com>
Desc: Merge pull request #23508 from 6a68/bug-1059332-fmd-error-strings

Bug 1059332 - Strings for FMD error flows r=arthurcc

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https://hg.mozilla.org/integration/gaia-central/rev/621b178274ef
Author: Jared Hirsch <ohai@6a68.net>
Desc: Bug 1059332 - Strings for FMD error flows. r=arthurcc
Bumping manifests a=b2g-bump