Point build badge to Github Actions

See gh-1508

Author: mhalbritter

.github/workflows/build.yml

@@ -3,6 +3,7 @@ on:
   push:
     branches:
       - main
+  workflow_dispatch:
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
   cancel-in-progress: true

README.adoc

@@ -1,4 +1,4 @@
-= Spring Initializr image:https://ci.spring.io/api/v1/teams/initializr/pipelines/initializr-0.20.x/jobs/build/badge["Build Status", link="https://ci.spring.io/teams/initializr/pipelines/initializr-0.20.x?groups=Build"] image:https://badges.gitter.im/spring-io/initializr.svg[link="https://gitter.im/spring-io/initializr?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge"]
+= Spring Initializr image:https://github.com/spring-io/initializr/actions/workflows/build.yml/badge.svg["Build status", link="https://github.com/spring-io/initializr/actions/workflows/build.yml"] image:https://badges.gitter.im/spring-io/initializr.svg[link="https://gitter.im/spring-io/initializr?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge"]
 
 :boot-doc: https://docs.spring.io/spring-boot/docs/current/reference/htmlsingle
 :code: https://github.com/spring-io/initializr/blob/main