deps: update dependency org.graalvm.buildtools:native-maven-plugin to v0.9.21

Author: renovate-bot

samples/native-image/pom.xml

@@ -130,7 +130,7 @@
  <plugin>
  <groupId>org.graalvm.buildtools</groupId>
  <artifactId>native-maven-plugin</artifactId>
- <version>0.9.20</version>
+ <version>0.9.21</version>
  <extensions>true</extensions>
  <configuration>
  <mainClass>com.example.spanner.NativeImageSpannerSample</mainClass>