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

@@            Coverage Diff            @@
##             master     #168   +/-   ##
=========================================
  Coverage     76.73%   76.73%           
  Complexity      579      579           
=========================================
  Files            42       42           
  Lines          3426     3426           
  Branches        237      237           
=========================================
  Hits           2629     2629           
  Misses          651      651           
  Partials        146      146           

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