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

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https://hg.mozilla.org/integration/gaia-central/rev/786f4b9e16b4
Author: Evyatar Amitay <evyatron@gmail.com>
Desc: Merge pull request #12745 from EverythingMe/924764-collection-design-changes

[924764] Redesign collection header and animations [r=crdlc]

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https://hg.mozilla.org/integration/gaia-central/rev/db0c04ba8a11
Author: Evyatar Amitay <evyatron@gmail.com>
Desc: [924764] Redesign collection header and animations [r=crdlc]