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

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https://hg.mozilla.org/integration/gaia-central/rev/21bd4186c994
Author: Yura Zenevich <yura.zenevich@gmail.com>
Desc: Merge pull request #24442 from yzen/bug-1071983

Bug 1071983 - fixing regression with hidden status bar icons when switch...

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https://hg.mozilla.org/integration/gaia-central/rev/51683d4ddc61
Author: Yura Zenevich <yzenevich@mozilla.com>
Desc: Bug 1071983 - fixing regression with hidden status bar icons when switching to previous app. r=etienne
Bumping manifests a=b2g-bump