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

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https://hg.mozilla.org/integration/gaia-central/rev/28b7819d09d2
Author: Michael Henretty <michael.henretty@gmail.com>
Desc: Merge pull request #18918 from mikehenrty/bug-1005721-fontsize

Bug 1005721 - reduce header font size from 2.5rem to 2.3rem

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https://hg.mozilla.org/integration/gaia-central/rev/5940f730c3f4
Author: Michael Henretty <michael.henretty@gmail.com>
Desc: Bug 1005721 - reduce header font size from 2.5rem to 2.3
Bumping manifests a=b2g-bump