Bump protobuf-java from 3.15.5 to 3.15.6

Bumps [protobuf-java](https://github.com/protocolbuffers/protobuf) from 3.15.5 to 3.15.6.
<details>
<summary>Release notes</summary>
<p><em>Sourced from <a href="https://github.com/protocolbuffers/protobuf/releases">protobuf-java's releases</a>.</em></p>
<blockquote>
<h2>Protocol Buffers v3.15.6</h2>
<h1>Ruby</h1>
<ul>
<li>Fixed bug in string comparison logic (<a href="https://github.com/protocolbuffers/protobuf/issues/8386">#8386</a>)</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a href="https://github.com/protocolbuffers/protobuf/commit/6aa539bf0195f188ff86efe6fb8bfa2b676cdd46"><code>6aa539b</code></a> Update protobuf version</li>
<li><a href="https://github.com/protocolbuffers/protobuf/commit/971a5935ed68e07a1e7e309cfcb4c39bfa87be8b"><code>971a593</code></a> Updated CHANGES.txt and PHP changelog for 3.15.6</li>
<li><a href="https://github.com/protocolbuffers/protobuf/commit/b2991b841981913e786269cf817c1e981aac0019"><code>b2991b8</code></a> Merge pull request <a href="https://github.com/protocolbuffers/protobuf/issues/8386">#8386</a> from haberman/ruby-string-compare</li>
<li><a href="https://github.com/protocolbuffers/protobuf/commit/bb322c2b39c1a98f78a5a433afe7a33cbae379a6"><code>bb322c2</code></a> [Ruby] Fixed bug in string comparison logic.</li>
<li>See full diff in <a href="https://github.com/protocolbuffers/protobuf/compare/v3.15.5...v3.15.6">compare view</a></li>
</ul>
</details>
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Fixes #837

COPYBARA_INTEGRATE_REVIEW=#837 from google:dependabot/maven/com.google.protobuf-protobuf-java-3.15.6 4e395bc
PiperOrigin-RevId: 362578804

pom.xml

@@ -39,7 +39,7 @@
     -->
     <guava.jre.version>30.1-jre</guava.jre.version>
     <gwt.version>2.9.0</gwt.version>
-    <protobuf.version>3.15.5</protobuf.version>
+    <protobuf.version>3.15.6</protobuf.version>
     <!-- Property for protobuf-lite protocArtifact, which isn't a "normal" Maven dep. -->
     <!-- TODO(cpovirk): Use protobuf.version instead. But that requires finding the new way to request the Lite runtime. -->
     <protobuf-lite.protoc.version>3.1.0</protobuf-lite.protoc.version>