[ plugin docs ] Update gradle plugin version to 5.0.0 (#8490)

* Update gradle plugin version to 4.3.1

1. Updated gradle plugin version to 4.3.1 (to keep in sync with maven sample)
2. Removed modelFilesConstrainedTo  in the gradle sample to generate code for dummies who are pasting and running.

* Update version to 5.0.0

Author: josetom

docs/plugins.md

@@ -15,7 +15,7 @@ Add to your `build->plugins` section (default phase is `generate-sources` phase)
 <plugin>
     <groupId>org.openapitools</groupId>
     <artifactId>openapi-generator-maven-plugin</artifactId>
-    <version>4.3.1</version>
+    <version>5.0.0</version>
     <executions>
         <execution>
             <goals>
@@ -76,7 +76,7 @@ buildscript {
     maven { url "https://repo1.maven.org/maven2" }
   }
   dependencies {
-    classpath "org.openapitools:openapi-generator-gradle-plugin:3.3.4"
+    classpath "org.openapitools:openapi-generator-gradle-plugin:5.0.0"
   }
 }
 
@@ -111,9 +111,6 @@ openApiGenerate {
     apiPackage = "org.openapi.example.api"
     invokerPackage = "org.openapi.example.invoker"
     modelPackage = "org.openapi.example.model"
-    modelFilesConstrainedTo = [
-            "Error"
-    ]
     configOptions = [
         dateLibrary: "java8"
     ]