Bumping gaia.json for 4 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-2_1/rev/4bf4e1814d0c
Author: Sherman Chen <shamenchens@users.noreply.github.com>
Desc: Merge pull request #24460 from shamenchens/Bug1071852-TaskManagerHomeButton

Bug 1071852 - Fix home button action in task manager, r=sfoster

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https://hg.mozilla.org/integration/gaia-2_1/rev/b21a6312f002
Author: Sherman Chen <shamen.chens@gmail.com>
Desc: Bug 1071852 - Fix home button action in task manager

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https://hg.mozilla.org/integration/gaia-2_1/rev/3d0d5ed3d44b
Author: David Flanagan <dflanagan@mozilla.com>
Desc: Merge pull request #24585 from davidflanagan/bug1074262

Bug 1074262: workaround graphics bug with will-change:opacity r=dmarcos

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https://hg.mozilla.org/integration/gaia-2_1/rev/a38e6f866b97
Author: David Flanagan <dflanagan@mozilla.com>
Desc: Bug 1074262: workaround graphics bug with will-change:opacity
Bumping manifests a=b2g-bump