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	3.12.0 -> 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)

v3.12.1

Compare Source

Ruby

• Re-add binary gems for Ruby 2.3 and 2.4. These are EOL upstream, however
  many people still use them and dropping support will require more
  coordination. (#​7529, #​7531).

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[codecov]

Codecov Report

Merging #220 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #220   +/-   ##
=========================================
  Coverage     67.62%   67.62%           
  Complexity      384      384           
=========================================
  Files            36       36           
  Lines          1986     1986           
  Branches        269      269           
=========================================
  Hits           1343     1343           
  Misses          535      535           
  Partials        108      108           

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