Unify OSS and commercial builds

[philwebb]

Gradle related tasks

• Add spring.build-type to gradle.properties
• Propagate root gradle.properties to buildSrc for access to all properties
• Allow custom release/milestone/snapshot repositories to be injected into Gradle build
• Allow custom release/milestone/snapshot repositories to be injected into XML files generated for tests
• Don't include MavenPomScm updates for commercial releases
• Update BOMR
• Don't publish home brew formula for commercial releases

GitHub Action related tasks

• Set commercial repository username/password environment variables when available
• Update changelog-generator to use correct repo and disable generate_links
• Use repository level variable for runs-on
• Disable pull request action
• Add commercial distribute workflow
• Template release.yml

Other tasks

• Update forward merge script to use correct repo
• Update documentation