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

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https://hg.mozilla.org/integration/gaia-2_1/rev/4e4603035839
Author: Florin Strugariu <florin.strugariu@softvision.ro>
Desc: Merge pull request #24289 from chirarobert/uplift_tap_back

Bug 1066530 - tap_back_button doesn't work with new gaia-header

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https://hg.mozilla.org/integration/gaia-2_1/rev/03c30203ace8
Author: Robert Chira <robertchira@P5075.(none)>
Desc: Bug 1066530 - tap_back_button doesn't work with new gaia-header
Bumping manifests a=b2g-bump