deps: update dependency io.github.java-diff-utils:java-diff-utils to v4.15 (#3430)

This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
|
[io.github.java-diff-utils:java-diff-utils](https://redirect.github.com/java-diff-utils/java-diff-utils)
| `4.12` -> `4.15` |
[![age](https://developer.mend.io/api/mc/badges/age/maven/io.github.java-diff-utils:java-diff-utils/4.15?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![adoption](https://developer.mend.io/api/mc/badges/adoption/maven/io.github.java-diff-utils:java-diff-utils/4.15?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![passing](https://developer.mend.io/api/mc/badges/compatibility/maven/io.github.java-diff-utils:java-diff-utils/4.12/4.15?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![confidence](https://developer.mend.io/api/mc/badges/confidence/maven/io.github.java-diff-utils:java-diff-utils/4.12/4.15?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|

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Author: renovate-bot

gapic-generator-java/pom.xml

@@ -473,7 +473,7 @@
     <dependency>
       <groupId>io.github.java-diff-utils</groupId>
       <artifactId>java-diff-utils</artifactId>
-      <version>4.12</version>
+      <version>4.15</version>
       <scope>test</scope>
     </dependency>
     <dependency>