How to use this project in 1.14.

There are some things you should change in build.gradle for getting 1.14's source code.

Firstly, you should change the mcp_config version. You can get the latest version of mcp_confg from here.

mcp {
    //From MinecraftForge build.gradle
    config = '1.14.4-20191210.153145'
    pipeline = 'client'
}

Secondly, you should change the mappings version and mcVersion. You can get the latest version of mappings files from here.

patcher {
    parent = project(':mcp')
    patchedSrc = file('src/main/java')
    mappings channel: 'snapshot', version: '20200119-1.14.4'
    mcVersion = '1.14.4'
    repositories {
      mavenLocal()
      maven { url = 'https://files.minecraftforge.net/maven' }
      jcenter()
      mavenCentral()
    }
}

Lastly, you should change start arguments.

task runclient(type: JavaExec) {
    group = "MCP"
    description = "Runs the client"
    doFirst {
        mkdir 'runclient'
    }
    classpath sourceSets.main.runtimeClasspath
    args = ['--accessToken', '0', '--version', '1.14.4', '--assetsDir', downloadAssets.output, '--assetIndex', '1.14']
    main 'net.minecraft.client.main.Main'
    workingDir 'runclient'
}