Add ascopes/protobuf-maven-plugin to third_party.md (#16656)

This PR adds my project at https://github.com/ascopes/protobuf-maven-plugin to the thirdparty README.

CLA signed in previous contributions to other Google projects on GitHub.

Closes #16656

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/16656 from ascopes:patch-1 6f8b43aa0d
PiperOrigin-RevId: 629411565

docs/third_party.md

@@ -187,6 +187,7 @@ Inactive:
         [![Maven Central](https://img.shields.io/maven-central/v/org.xolstice.maven.plugins/protobuf-maven-plugin.svg)](https://repo1.maven.org/maven2/org/xolstice/maven/plugins/protobuf-maven-plugin/)
     *   https://code.google.com/p/maven-protoc-plugin/
     *   https://github.com/os72/protoc-jar-maven-plugin
+    *   https://ascopes.github.io/protobuf-maven-plugin
 *   [Protobuf Plugin for Gradle](https://github.com/google/protobuf-gradle-plugin)
 *   [Sbt plugin for Protocol Buffers](https://github.com/Atry/sbt-cppp)