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

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https://hg.mozilla.org/integration/gaia-central/rev/2161df9eb0f3
Author: gitmai <mri@tid.es>
Desc: Merge pull request #21588 from empoalp/bug_963361

Bug 963361 - Usage app shows too much white space at the bottom
r=vingtetun

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https://hg.mozilla.org/integration/gaia-central/rev/2ac4cfb4fdb4
Author: David Garcia <davidg@tid.es>
Desc: Bug 963361 - Usage app shows too much white space at the bottom

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https://hg.mozilla.org/integration/gaia-central/rev/a3ee990e1037
Author: Arnau <arnau@arnaumarch.com>
Desc: Merge pull request #21348 from pacorampas/1033970

Bug 1033970 - In multicall, Phone with the background when you click on it r=rik

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https://hg.mozilla.org/integration/gaia-central/rev/558c2a44046c
Author: Paco Rampas <pacorampas@gmail.com>
Desc: Bug 1033970 - Line not complete highlighted when tapping on a participant in a multiparty call
Bumping manifests a=b2g-bump