Adjust FCM JSON config for getting token

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Project coverage is 75.75%. Comparing base (55586b2) to head (a026143).

Files	Patch %	Lines
handlers/firebase/handler.go	0.00%	5 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #772 +/- ## ======================================= Coverage 75.74% 75.75% ======================================= Files 110 110 Lines 10420 10419 -1 ======================================= Hits 7893 7893 + Misses 1803 1802 -1 Partials 724 724 ```

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