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

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https://hg.mozilla.org/integration/gaia-central/rev/6fe7a90a749b
Author: Michael Henretty <michael.henretty@gmail.com>
Desc: Merge pull request #22001 from mikehenrty/bug-1038988-wallpaper-flatten

Bug 1038988 - Use fast path to GPU for wallpaper

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https://hg.mozilla.org/integration/gaia-central/rev/9e5fd056d153
Author: Michael Henretty <michael.henretty@gmail.com>
Desc: Bug 1038988 - Use fast path to GPU for wallpaper
Bumping manifests a=b2g-bump