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

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https://hg.mozilla.org/integration/gaia-central/rev/4ef8c5cf2eef
Author: Sherman Chen <shamenchens@users.noreply.github.com>
Desc: Merge pull request #21976 from shamenchens/Bug1034956-RemoveTabletSilenceIncomingCall-2

Bug1034956 remove tablet silence incoming call 2, r=alive

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https://hg.mozilla.org/integration/gaia-central/rev/4eee4a005153
Author: Sherman Chen <shamen.chens@gmail.com>
Desc: Bug 1034956 - Remove silence incoming call on tablet power menu
Bumping manifests a=b2g-bump