25311: [Build] Upgraded to Gradle 6.5.

This includes the following changes:
- Upgraded to Gradle 6.5 everywhere, except for the IntelliJ plugin (it uses Gradle 5.6.2 for now due to warnings when using Gradle 6.x)
- Adjusted the Jenkins plugin to use Gradle-style dependencies; see jenkinsci/gradle-jpi-plugin#139
- Fixed the Jenkins makefile to run CheckStyle for every build and resolved all existing violations
- Fixed repository URLs (Nexus) to use https instead of http

Reviewed by @reniers.

build.gradle

@@ -11,9 +11,9 @@ intellij {
     // Specifying these custom repositories enables us to cache them within TIOBE.
     // If this causes trouble, either make sure you are in the TIOBE network, or
     // comment out these lines temporarily.
-    intellijRepo 'http://artifacts.tiobe.com/repository/maven/'
-    pluginsRepo 'http://artifacts.tiobe.com/repository/maven/'
-    jreRepo 'http://artifacts.tiobe.com/repository/intellij-jdk/'
+    intellijRepo 'https://artifacts.tiobe.com/repository/maven/'
+    pluginsRepo 'https://artifacts.tiobe.com/repository/maven/'
+    jreRepo 'https://artifacts.tiobe.com/repository/intellij-jdk/'
 }
 
 patchPluginXml {
@@ -30,7 +30,7 @@ repositories {
     // If this causes trouble, either make sure you are in the TIOBE network, or
     // add the repository mavenCentral().
     maven {
-        url 'http://artifacts.tiobe.com/repository/maven/'
+        url 'https://artifacts.tiobe.com/repository/maven/'
     }
     //mavenCentral()
 }

settings.gradle

@@ -4,7 +4,7 @@ pluginManagement {
         // If this causes trouble, either make sure you are in the TIOBE network, or
         // comment out these lines temporarily.
         maven {
-            url 'http://artifacts.tiobe.com/repository/maven/'
+            url 'https://artifacts.tiobe.com/repository/maven/'
         }
     }
 }