[gradle] consistent use of maven url in gradle files (#5045)

* wrap maven url with uri function

* consistent use of maven url in gradle files

modules/openapi-generator-gradle-plugin/README.adoc

@@ -56,7 +56,7 @@ Using https://docs.gradle.org/current/userguide/plugins.html#sec:old_plugin_appl
 buildscript {
   repositories {
     mavenLocal()
-    maven { url = "https://repo1.maven.org/maven2" }
+    maven { url "https://repo1.maven.org/maven2" }
     // or, via Gradle Plugin Portal:
     // url "https://plugins.gradle.org/m2/"
   }

modules/openapi-generator-gradle-plugin/build.gradle

@@ -2,7 +2,7 @@ buildscript {
     ext.kotlin_version = '1.3.20'
     repositories {
         mavenLocal()
-        maven { url = "https://repo1.maven.org/maven2" }
+        maven { url "https://repo1.maven.org/maven2" }
         maven {
             url "https://plugins.gradle.org/m2/"
         }
@@ -46,7 +46,7 @@ targetCompatibility = 1.8
 
 repositories {
     jcenter()
-    maven { url = "https://repo1.maven.org/maven2" }
+    maven { url "https://repo1.maven.org/maven2" }
     mavenLocal()
     maven {
         url "https://oss.sonatype.org/content/repositories/releases/"

modules/openapi-generator-gradle-plugin/samples/local-spec/build.gradle

@@ -1,7 +1,7 @@
 buildscript {
     repositories {
         mavenLocal()
-        maven { url = "https://repo1.maven.org/maven2" }
+        maven { url "https://repo1.maven.org/maven2" }
         maven {
             url "https://plugins.gradle.org/m2/"
         }