Downgrade com.github.kongchen:swagger-maven-plugin to 3.1.8 because ApiParams are shown as required always (flowable#4082)

Author: WelschChristopher

docs/public-api/tools/flowable-oas-generator/pom.xml

@@ -44,7 +44,7 @@
             <plugin>
                 <groupId>com.github.kongchen</groupId>
                 <artifactId>swagger-maven-plugin</artifactId>
-                <version>3.1.8</version>
+                <version>3.1.7</version>
                 <configuration>
                     <apiSources>
                         <apiSource>

modules/flowable-app-rest/pom.xml

@@ -125,7 +125,7 @@
                     <plugin>
                         <groupId>com.github.kongchen</groupId>
                         <artifactId>swagger-maven-plugin</artifactId>
-                        <version>3.1.8</version>
+                        <version>3.1.7</version>
                         <configuration>
                             <apiSources>
                                 <apiSource>