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

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https://hg.mozilla.org/integration/gaia-central/rev/2d20953fd97b
Author: Aleh Zasypkin <aleh.zasypkin@gmail.com>
Desc: Merge pull request #22315 from azasypkin/bug-1046248-attach-button-size

Bug 1046248 - [Messages] Set explicit background image size for "Add attachment" button. r=schung

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https://hg.mozilla.org/integration/gaia-central/rev/f70a14767456
Author: Aleh Zasypkin <azasypkin@mozilla.com>
Desc: Bug 1046248 - [Messages] Set explicit background image size for "Add attachment" button. r=schung
Bumping manifests a=b2g-bump