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

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https://hg.mozilla.org/integration/gaia-central/rev/8b4930ac91ec
Author: evanxd <itoyxd@gmail.com>
Desc: Merge pull request #23172 from evanxd/bug-1057194

Bug 1057194 - navigator.mozHour12 should not have the default value. r=archurcc

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https://hg.mozilla.org/integration/gaia-central/rev/bff831643b99
Author: Evan Xd <itoyxd@gmail.com>
Desc: Bug 1057194 - navigator.mozHour12 should not have a default value.
Bumping manifests a=b2g-bump