Bump log4j-core from 2.14.1 to 2.15.0 in /samples/spring/pet-store

Bumps log4j-core from 2.14.1 to 2.15.0.

---
updated-dependencies:
- dependency-name: org.apache.logging.log4j:log4j-core
  dependency-type: direct:production
...

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Author: dependabot[bot]

samples/spring/pet-store/pom.xml

@@ -28,7 +28,7 @@
         <maven.compiler.target>1.8</maven.compiler.target>
         <spring.version>5.3.9</spring.version>
         <junit.version>4.13.2</junit.version>
-        <log4j.version>2.14.1</log4j.version>
+        <log4j.version>2.15.0</log4j.version>
     </properties>
 
     <dependencies>