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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