1.8.9 Upgrading from gradle 4.5 and forgegradle "2.1-SNAPSHOT" to a newer version of both

[gwoonryan]

Hi, I need some help upgrading my Gradle and ForgeGradle versions for my 1.8.9 mod.
I just started modding for forge. I do have quite some experience with coding in Java, but I always used the Maven build system instead of Gradle, so this is still quite hard for me.
If I load my "build.gradle" it tells me that I need to / that it is better to upgrade my ForgeGradle to a 5.x version, I guess that I need to upgrade my Gradle then also? (IntelliJ was already mad that I was using this old of a version).

Can someone maybe help me with this because I don't even really know where to start...
(I tried just upgrading Gradle and changing the version of ForgeGradle, but I couldn't get anything working)

This is my last working build.gradle (for gradle 4.5 and ForgeGradle 2.1-SNAPSHOT):

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// For those who want the bleeding edge
buildscript {
    repositories {
        jcenter()
        maven {
            name = "forge"
            url = "http://files.minecraftforge.net/maven"
        }
    }
    dependencies {
        classpath 'net.minecraftforge.gradle:ForgeGradle:2.1-SNAPSHOT'
    }
}
apply plugin: 'net.minecraftforge.gradle.forge'

version = "1.1"
group= "me.ryan.ryanair"
archivesBaseName = "ryanair"

minecraft {
    version = "1.8.9-11.15.1.2318-1.8.9"
    runDir = "run"
    mappings = "stable_20"
}

dependencies {

}

processResources
        {
            // this will ensure that this task is redone when the versions change.
            inputs.property "version", project.version
            inputs.property "mcversion", project.minecraft.version

            // replace stuff in mcmod.info, nothing else
            from(sourceSets.main.resources.srcDirs) {
                include 'mcmod.info'

                // replace version and mcversion
                expand '1.1':project.version, '1.8.9':project.minecraft.version
            }

            // copy everything else, thats not the mcmod.info
            from(sourceSets.main.resources.srcDirs) {
                exclude 'mcmod.info'
            }
        }

task copyJarToBin(type: Copy) {
    from "path\\to\\my\\modding\\folder\\Ryanair\\build\\libs\\ryanair-1.1.jar"
    into "path\\to\\my\\mc\\instance\\mods"
}