firestore: bump grpc from 1.20.3 to 1.22.2

[CapOM]

grpc 1.22.x is the only grpc release providing binaries
for electron 4.2.

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[CapOM]

I signed it!

[googlebot]

CLAs look good, thanks!

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[CapOM]

Hi @hsubox76 , please take a look (sorry not sure who can review it), thx!

[Feiyang1]

Thank so much for the contribution! The dep has been updated in #1962