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

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https://hg.mozilla.org/integration/gaia-central/rev/21f9169d963a
Author: pacorampas <b.frv@tid.es>
Desc: Merge pull request #24666 from pacorampas/977588

Bug 977588 - [Dialer] The Active call screen does not have the option to place calls on hold. r=drs

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https://hg.mozilla.org/integration/gaia-central/rev/f85d56ee7c10
Author: Paco Rampas <pacorampas@gmail.com>
Desc: Bug 977588 - [Dialer] The Active call screen does not have the option to place calls on hold.
Bumping manifests a=b2g-bump