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

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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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