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

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https://hg.mozilla.org/integration/gaia-central/rev/b20987909575
Author: Arnau <arnau@arnaumarch.com>
Desc: Merge pull request #20876 from pacorampas/aligment-buttons-oncall-1028126

Bug 1028126 - [Dialer][Call screen] In incoming call the buttons haven't a good alignment r=etienne

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https://hg.mozilla.org/integration/gaia-central/rev/902ef029096d
Author: Paco Rampas <pacorampas@gmail.com>
Desc: Bug 1028126 - [Dialer][Call screen] In incoming call the buttons haven't a good alignment
Bumping manifests a=b2g-bump