Backport rest of build related changes to 1.2.x

[simonbasle]

Complements the partial backport done in 9b7b6b8 with other changes to the build (Gradle version, switching to maven-publish, artifactory)

Cherry-pick of 04fd411, 39d991a, 6483214, cf3056f, 65124c8, 0dbfeba and 35e0ae7.

• [build] Bump/polish Gradle, add maven-publish and artifactory (#115)

  • bumped Gradle to 5.6.4
  • removed propdeps (no optional dependency anyway)
  • removed outdated wrapper tasks and variables
  • externalized $reactorCoreVersion to gradle.properties
  • applied setup.gradle from inside configure(allProjects)
  • now applies maven-publish and artifactory plugins
  • removed useless classpath printing task
  • modified how the pdf is renamed when zipping docs (so that it is renamed late and can pick up a custom version)
  • minor polish of deprecated constructs / warnings

• [build] Add releaser.gradle with groupId helper task (#115)

• [build] Ensure rootProject.name is reactor-rabbitmq

• Conditionnally add mavenLocal when releaser script is applied (#117) This is only activated when passing a non-false -PreleaserDryRun to Gradle.

• fix #119 Ensure docsZip defines a -docs classifier

• [build] Add bumpVersionsInReadme, better Artifactory creds, buildId

  • Added a bumpVersionsInReadme task that needs to be explicitly executed with oldVersion, currentVersion and nextVersion properties
  • For Artifactory, we declare an explicit credentials block using alternate properties like artifactory_publish_xxx, that can be set via an env var of the form ORG_GRADLE_PROJECT_artifactory_publish_xxx.
  • We give the build info a meaningful name and id and ensure we don't publish environment variables.

• [build] Fix Bamboo publications by removing onlyIf clause