Bumping gaia.json for 6 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/f17cd16252b9
Author: Jonathan Griffin <jgriffin@mozilla.com>
Desc: Merge pull request #16335 from jonallengriffin/0216

Bug 973121 - Bump gaiatest to 0.21.6, r=wlach

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https://hg.mozilla.org/integration/gaia-central/rev/e5ec4b2a19d2
Author: Jonathan Griffin <jgriffin@mozilla.com>
Desc: Bug 973121 - Bump gaiatest to 0.21.6, r=wlach

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https://hg.mozilla.org/integration/gaia-central/rev/fa1d16c4eee1
Author: vingtetun <21@vingtetun.org>
Desc: Merge pull request #16333 from vingtetun/bug972957-v2

Bug 972957 - [Messages] Remove some css warnings. r=rwaldron

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https://hg.mozilla.org/integration/gaia-central/rev/c3c62ece840a
Author: Vivien Nicolas <21@vingtetun.org>
Desc: Bug 972957 - [Messages] Remove some css warnings. r=rwaldron

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https://hg.mozilla.org/integration/gaia-central/rev/737930e7b014
Author: vingtetun <21@vingtetun.org>
Desc: Merge pull request #16332 from vingtetun/bug969827

Bug 969827 - Map a preference to a Gaia setting to configure the jank th...

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https://hg.mozilla.org/integration/gaia-central/rev/8d882656408e
Author: Vivien Nicolas <21@vingtetun.org>
Desc: Bug 969827 - Map a preference to a Gaia setting to configure the jank threshold. r=janx
Bumping manifests a=b2g-bump