Revert addition of JDK-12 based CI

See gh-15031

ci/images/spring-boot-jdk12-ci-image/Dockerfile

-FROM openjdk:12-ea-17-jdk
-
-RUN apt-get update && \
-    apt-get install -y git && \
-    apt-get install -y libxml2-utils && \
-    apt-get install -y jq
-
-ADD https://raw.githubusercontent.com/spring-io/concourse-java-scripts/v0.0.2/concourse-java.sh /opt/
-
-ENV DOCKER_VERSION=17.05.0-ce \
-    ENTRYKIT_VERSION=0.4.0
-
-RUN apt-get update && \
-    apt-get install -y curl && \
-	apt-get install -y libudev1 && \
-	apt-get install -y iptables && \
-    curl https://get.docker.com/builds/Linux/x86_64/docker-${DOCKER_VERSION}.tgz | tar zx && \
-    mv /docker/* /bin/ && chmod +x /bin/docker*
-
-# Install entrykit
-RUN curl -L https://github.com/progrium/entrykit/releases/download/v${ENTRYKIT_VERSION}/entrykit_${ENTRYKIT_VERSION}_Linux_x86_64.tgz | tar zx && \
-    chmod +x entrykit && \
-    mv entrykit /bin/entrykit && \
-    entrykit --symlink
-
-ADD https://raw.githubusercontent.com/spring-projects/spring-boot/master/ci/images/docker-lib.sh /docker-lib.sh
-
-ENTRYPOINT [ \
-	"switch", \
-		"shell=/bin/sh", "--", \
-	"codep", \
-		"/bin/docker daemon" \
-]
\ No newline at end of file

ci/pipeline.yml

@@ -73,13 +73,6 @@ resources:
     username: ((docker-hub-username))
     password: ((docker-hub-password))
     tag: ((branch))
-- name: spring-boot-jdk12-ci-image
-  type: docker-image
-  source:
-    repository: ((docker-hub-organization))/spring-boot-jdk12-ci-image
-    username: ((docker-hub-username))
-    password: ((docker-hub-password))
-    tag: ((branch))
 - name: artifactory-repo
   type: artifactory-resource
   source:
@@ -101,13 +94,6 @@ resources:
     access_token: ((github-access-token))
     branch: ((branch))
     context: jdk11-build
-- name: repo-status-jdk12-build
-  type: github-status-resource
-  source:
-    repository: ((github-repo-name))
-    access_token: ((github-access-token))
-    branch: ((branch))
-    context: jdk12-build
 - name: slack-alert
   type: slack-notification
   source:
@@ -123,9 +109,6 @@ jobs:
   - put: spring-boot-jdk11-ci-image
     params:
       build: ci-images-git-repo/ci/images/spring-boot-jdk11-ci-image
-  - put: spring-boot-jdk12-ci-image
-    params:
-      build: ci-images-git-repo/ci/images/spring-boot-jdk12-ci-image
 - name: build
   serial: true
   public: true
@@ -274,52 +257,6 @@ jobs:
       silent: true
       icon_emoji: ":concourse:"
       username: concourse-ci
-- name: jdk12-build
-  serial: true
-  public: true
-  plan:
-  - get: spring-boot-jdk12-ci-image
-  - get: git-repo
-    trigger: true
-  - put: repo-status-jdk12-build
-    params: { state: "pending", commit: "git-repo" }
-  - do:
-    - task: build-project
-      privileged: true
-      timeout: 1h30m
-      image: spring-boot-jdk12-ci-image
-      file: git-repo/ci/tasks/build-project.yml
-    - aggregate:
-      - task: build-samples
-        timeout: 1h30m
-        image: spring-boot-jdk12-ci-image
-        file: git-repo/ci/tasks/build-samples.yml
-      - task: build-integration-tests
-        timeout: 1h30m
-        image: spring-boot-jdk12-ci-image
-        file: git-repo/ci/tasks/build-integration-tests.yml
-      - task: build-deployment-tests
-        timeout: 1h30m
-        image: spring-boot-jdk12-ci-image
-        file: git-repo/ci/tasks/build-deployment-tests.yml
-    on_failure:
-      do:
-      - put: repo-status-jdk12-build
-        params: { state: "failure", commit: "git-repo" }
-      - put: slack-alert
-        params:
-          text: ":concourse-failed: <https://ci.spring.io/teams/${BUILD_TEAM_NAME}/pipelines/${BUILD_PIPELINE_NAME}/jobs/${BUILD_JOB_NAME}/builds/${BUILD_NAME}|${BUILD_PIPELINE_NAME} ${BUILD_JOB_NAME} failed!>"
-          silent: true
-          icon_emoji: ":concourse:"
-          username: concourse-ci
-  - put: repo-status-jdk12-build
-    params: { state: "success", commit: "git-repo" }
-  - put: slack-alert
-    params:
-      text: ":concourse-succeeded: <https://ci.spring.io/teams/${BUILD_TEAM_NAME}/pipelines/${BUILD_PIPELINE_NAME}/jobs/${BUILD_JOB_NAME}/builds/${BUILD_NAME}|${BUILD_PIPELINE_NAME} ${BUILD_JOB_NAME} was successful!>"
-      silent: true
-      icon_emoji: ":concourse:"
-      username: concourse-ci
 - name: stage-milestone
   serial: true
   plan:
@@ -500,7 +437,7 @@ jobs:
       BINTRAY_REPO: ((bintray-repo))
 groups:
 - name: "Build"
-  jobs: ["build", "jdk11-build", "jdk12-build"]
+  jobs: ["build", "jdk11-build"]
 - name: "Release"
   jobs: ["stage-milestone", "stage-rc", "stage-release", "promote-milestone", "promote-rc", "promote-release", "sync-to-maven-central"]
 - name: "CI Images"