deps: update dependency com.google.protobuf:protobuf-java to v3.12.0

[renovate-bot]

This PR contains the following updates:

Package	Update	Change
com.google.protobuf:protobuf-java	minor	3.11.4 -> 3.12.0

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[codecov]

Codecov Report

Merging #188 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #188      +/-   ##
============================================
+ Coverage     78.88%   78.92%   +0.03%     
  Complexity      302      302              
============================================
  Files            21       21              
  Lines          2719     2719              
  Branches        150      150              
============================================
+ Hits           2145     2146       +1     
  Misses          513      513              
+ Partials         61       60       -1     

Impacted Files	Coverage Δ	Complexity Δ
.../com/google/cloud/pubsub/v1/MessageDispatcher.java	85.64% <0.00%> (+0.49%)	25.00% <0.00%> (ø%)

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