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

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https://hg.mozilla.org/integration/gaia-central/rev/28a6ae427098
Author: Anthony Ricaud <anthony@ricaud.me>
Desc: Merge pull request #17140 from Rik/voicemail-hotkey-978114

Bug 978114 - [DSDS] Voicemail hotkey should respect the SIM preference r=etienne

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https://hg.mozilla.org/integration/gaia-central/rev/6c655acc0255
Author: Anthony Ricaud <anthony@ricaud.me>
Desc: Bug 978114 - [DSDS] Voicemail hotkey should respect the SIM preference
Bumping manifests a=b2g-bump