Add separate parameter for milestone name

Since the branch name doesn't always correspond to the milestone name,
eg, master.

See gh-20680

ci/parameters.yml

@@ -6,6 +6,7 @@ github-repo-name: "spring-projects/spring-boot"
 docker-hub-organization: "springci"
 artifactory-server: "https://repo.spring.io"
 branch: "2.1.x"
+milestone: "2.1.x"
 build-name: "spring-boot"
 pipeline-name: "spring-boot-2.1.x"
 concourse-url: "https://ci.spring.io"

ci/pipeline.yml

@@ -161,7 +161,7 @@ jobs:
         GITHUB_PASSWORD: ((github-password))
         GITHUB_USERNAME: ((github-username))
         JDK_VERSION: java8
-        MILESTONE: ((branch))
+        MILESTONE: ((milestone))
       image: spring-boot-ci-image
     - task: detect-jdk11-update
       file: git-repo/ci/tasks/detect-jdk-updates.yml
@@ -171,7 +171,7 @@ jobs:
         GITHUB_PASSWORD: ((github-password))
         GITHUB_USERNAME: ((github-username))
         JDK_VERSION: java11
-        MILESTONE: ((branch))
+        MILESTONE: ((milestone))
       image: spring-boot-ci-image
 - name: detect-ubuntu-image-updates
   plan:
@@ -187,7 +187,7 @@ jobs:
         GITHUB_ORGANIZATION: spring-projects
         GITHUB_PASSWORD: ((github-password))
         GITHUB_USERNAME: ((github-username))
-        MILESTONE: ((branch))
+        MILESTONE: ((milestone))
       image: spring-boot-ci-image
 - name: detect-docker-updates
   plan:
@@ -202,7 +202,7 @@ jobs:
       params:
         GITHUB_REPO: spring-boot
         GITHUB_ORGANIZATION: spring-projects
-        MILESTONE: ((branch))
+        MILESTONE: ((milestone))
       image: spring-boot-ci-image
     - put: git-repo-ci-docker
       params: