deps: update dependency com.google.protobuf:protobuf-bom to v3.12.2 (#168)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [com.google.protobuf:protobuf-bom](https://developers.google.com/protocol-buffers/) ([source](https://togithub.com/protocolbuffers/protobuf)) | patch | `3.12.0` -> `3.12.2` |

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### Release Notes

<details>
<summary>protocolbuffers/protobuf</summary>

### [`v3.12.2`](https://togithub.com/protocolbuffers/protobuf/releases/v3.12.2)

[Compare Source](https://togithub.com/protocolbuffers/protobuf/compare/v3.12.1...v3.12.2)

##### C++

-   Simplified the template export macros to fix the build for mingw32. ([#&#8203;7539](https://togithub.com/protocolbuffers/protobuf/issues/7539))

##### Objective-C

-   Fix for the :protobuf_objc target in the Bazel BUILD file. ([#&#8203;7538](https://togithub.com/protocolbuffers/protobuf/issues/7538))

### [`v3.12.1`](https://togithub.com/protocolbuffers/protobuf/releases/v3.12.1)

[Compare Source](https://togithub.com/protocolbuffers/protobuf/compare/v3.12.0...v3.12.1)

### 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. ([#&#8203;7529](https://togithub.com/protocolbuffers/protobuf/issues/7529), [#&#8203;7531](https://togithub.com/protocolbuffers/protobuf/issues/7531)).

</details>

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Author: renovate-bot

pom.xml

@@ -68,7 +68,7 @@
     <google.common-protos.version>1.18.0</google.common-protos.version>
     <gax.version>1.56.0</gax.version>
     <grpc.version>1.29.0</grpc.version>
-    <protobuf.version>3.12.0</protobuf.version>
+    <protobuf.version>3.12.2</protobuf.version>
     <junit.version>4.13</junit.version>
     <guava.version>29.0-android</guava.version>
     <threeten.version>1.4.4</threeten.version>