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

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https://hg.mozilla.org/integration/gaia-central/rev/a1ecdfbd4d96
Author: Timothy Guan-tin Chien <timdream@gmail.com>
Desc: Revert "Merge pull request #13852 from bgirard/HomescrenBackground"

This reverts commit d14af806d87a2ca6098ca0a011256cb3b1d4cf9a, reversing
changes made to 5e978dcebd7d122fd11988467c0d300dc51494c3.