deps: update dependency com.google.protobuf:protobuf-bom to v3.12.2

[renovate-bot]

This PR contains the following updates:

Package	Update	Change
com.google.protobuf:protobuf-bom (source)	patch	3.12.1 -> 3.12.2

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Release Notes

protocolbuffers/protobuf

v3.12.2

Compare Source

C++

• Simplified the template export macros to fix the build for mingw32. (#​7539)

Objective-C

• Fix for the :protobuf_objc target in the Bazel BUILD file. (#​7538)

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[codecov]

Codecov Report

Merging #196 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #196   +/-   ##
=========================================
  Coverage     68.98%   68.98%           
  Complexity      219      219           
=========================================
  Files            23       23           
  Lines          1654     1654           
  Branches         46       46           
=========================================
  Hits           1141     1141           
  Misses          468      468           
  Partials         45       45           

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