feat: upgrade gcm to fcm

[appleboy]

https://github.com/edganiukov/fcm

fix #230

[codecov-io]

Codecov Report

Merging #231 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #231      +/-   ##
==========================================
+ Coverage   96.22%   96.24%   +0.01%     
==========================================
  Files          13       13              
  Lines         928      932       +4     
==========================================
+ Hits          893      897       +4     
  Misses         27       27              
  Partials        8        8

Impacted Files	Coverage Δ
gorush/notification.go	91.22% <100%> (+0.13%)	⬆️

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