feat(codebuild): rename standard 1.0 image and add standard 2.0 image

Fixes aws#2306

packages/@aws-cdk/aws-codebuild/lib/project.ts

@@ -629,7 +629,7 @@ export class Project extends ProjectBase {
       props.cacheBucket.grantReadWrite(this.role);
     }
 
-    this.buildImage = (props.environment && props.environment.buildImage) || LinuxBuildImage.UBUNTU_18_04_STANDARD_1_0;
+    this.buildImage = (props.environment && props.environment.buildImage) || LinuxBuildImage.STANDARD_1_0;
 
     // let source "bind" to the project. this usually involves granting permissions
     // for the code build role to interact with the source.
@@ -951,7 +951,7 @@ export interface BuildEnvironment {
   /**
    * The image used for the builds.
    *
-   * @default LinuxBuildImage.UBUNTU_18_04_STANDARD_1_0
+   * @default LinuxBuildImage.STANDARD_1_0
    */
   readonly buildImage?: IBuildImage;
 
@@ -1033,7 +1033,7 @@ export interface IBuildImage {
  * @see https://docs.aws.amazon.com/codebuild/latest/userguide/build-env-ref-available.html
  */
 export class LinuxBuildImage implements IBuildImage {
-  public static readonly UBUNTU_18_04_STANDARD_1_0 = new LinuxBuildImage('aws/codebuild/standard:1.0');
+  public static readonly STANDARD_1_0 = new LinuxBuildImage('aws/codebuild/standard:1.0');
   public static readonly UBUNTU_14_04_BASE = new LinuxBuildImage('aws/codebuild/ubuntu-base:14.04');
   public static readonly UBUNTU_14_04_ANDROID_JAVA8_24_4_1 = new LinuxBuildImage('aws/codebuild/android-java-8:24.4.1');
   public static readonly UBUNTU_14_04_ANDROID_JAVA8_26_1_1 = new LinuxBuildImage('aws/codebuild/android-java-8:26.1.1');