deps: update dependency com.google.protobuf:protobuf-bom to v3.15.6 (#…

…122)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [com.google.protobuf:protobuf-bom](https://developers.google.com/protocol-buffers/) ([source](https://togithub.com/protocolbuffers/protobuf)) | `3.15.2` -> `3.15.6` | [![age](https://badges.renovateapi.com/packages/maven/com.google.protobuf:protobuf-bom/3.15.6/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.protobuf:protobuf-bom/3.15.6/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.protobuf:protobuf-bom/3.15.6/compatibility-slim/3.15.2)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.protobuf:protobuf-bom/3.15.6/confidence-slim/3.15.2)](https://docs.renovatebot.com/merge-confidence/) |

---

### Release Notes

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

### [`v3.15.6`](https://togithub.com/protocolbuffers/protobuf/releases/v3.15.6)

[Compare Source](https://togithub.com/protocolbuffers/protobuf/compare/v3.15.5...v3.15.6)

### Ruby

-   Fixed bug in string comparison logic ([#&#8203;8386](https://togithub.com/protocolbuffers/protobuf/issues/8386))

### [`v3.15.5`](https://togithub.com/protocolbuffers/protobuf/releases/v3.15.5)

[Compare Source](https://togithub.com/protocolbuffers/protobuf/compare/v3.15.4...v3.15.5)

### Ruby

-   Fixed quadratic memory use in array append ([#&#8203;8379](https://togithub.com/protocolbuffers/protobuf/issues/8379))

### PHP

-   Fixed quadratic memory use in array append ([#&#8203;8379](https://togithub.com/protocolbuffers/protobuf/issues/8379))

### C++

-   Do not disable RTTI by default in the CMake build ([#&#8203;8377](https://togithub.com/protocolbuffers/protobuf/issues/8377))

### [`v3.15.4`](https://togithub.com/protocolbuffers/protobuf/releases/v3.15.4)

[Compare Source](https://togithub.com/protocolbuffers/protobuf/compare/v3.15.3...v3.15.4)

### Ruby

-   Fixed SEGV when users pass nil messages ([#&#8203;8363](https://togithub.com/protocolbuffers/protobuf/issues/8363))
-   Fixed quadratic memory usage when appending to arrays ([#&#8203;8364](https://togithub.com/protocolbuffers/protobuf/issues/8364))

### C++

-   Create a CMake option to control whether or not RTTI is enabled ([#&#8203;8361](https://togithub.com/protocolbuffers/protobuf/issues/8361))

### PHP

-   read_property() handler is not supposed to return NULL ([#&#8203;8362](https://togithub.com/protocolbuffers/protobuf/issues/8362))

### [`v3.15.3`](https://togithub.com/protocolbuffers/protobuf/releases/v3.15.3)

[Compare Source](https://togithub.com/protocolbuffers/protobuf/compare/v3.15.2...v3.15.3)

### Ruby

-   Ruby &lt;2.7 now uses WeakMap too, which prevents memory leaks. ([#&#8203;8341](https://togithub.com/protocolbuffers/protobuf/issues/8341))

</details>

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pom.xml

@@ -64,7 +64,7 @@
     <github.global.server>github</github.global.server>
     <site.installationModule>google-iam-parent</site.installationModule>
     <google.common-protos.version>1.17.0</google.common-protos.version>
-    <protobuf.version>3.15.2</protobuf.version>
+    <protobuf.version>3.15.6</protobuf.version>
     <junit.version>4.13.2</junit.version>
     <grpc.version>1.36.0</grpc.version>
     <guava.version>30.1-android</guava.version>