fix: samples/server/petstore/kotlin-springboot-bigdecimal-default/pom.xml to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JAVA-COMFASTERXMLJACKSONCORE-10500754

Author: snyk-bot

samples/server/petstore/kotlin-springboot-bigdecimal-default/pom.xml

@@ -6,7 +6,7 @@
     <name>openapi-spring</name>
     <version>1.0.0</version>
     <properties>
-        <springdoc-openapi.version>1.6.8</springdoc-openapi.version>
+        <springdoc-openapi.version>1.7.0</springdoc-openapi.version>
         <findbugs-jsr305.version>3.0.2</findbugs-jsr305.version>
         <javax-annotation.version>1.3.2</javax-annotation.version>
         <kotlin-test-junit5.version>1.6.21</kotlin-test-junit5.version>