Add publish_release lane

Base off woocommerce/woocommerce-android#12450

fastlane/lanes/release.rb

@@ -183,6 +183,59 @@
     end
   end
 
+  lane :publish_release do |skip_confirm: false|
+    ensure_git_status_clean
+    ensure_git_branch_is_release_branch!
+
+    version_number = release_version_current
+
+    current_branch = release_branch_name(release_version: version_number)
+    next_release_branch = release_branch_name(release_version: release_version_next)
+
+    UI.important <<~PROMPT
+      Publish the #{version_number} release. This will:
+      - Publish the existing draft `#{version_number}` release on GitHub
+      - Which will also have GitHub create the associated Git tag, pointing to the tip of #{current_branch}
+      - If the release branch for the next version `#{next_release_branch}` already exists, backmerge `#{current_branch}` into it
+      - If needed, backmerge `#{current_branch}` back into `#{DEFAULT_BRANCH}`
+      - Delete the `#{current_branch}` branch
+    PROMPT
+    UI.user_error!("Terminating as requested. Don't forget to run the remainder of this automation manually.") unless skip_confirm || UI.confirm('Do you want to continue?')
+
+    UI.important "Publishing release #{version_number} on GitHub..."
+
+    publish_github_release(
+      repository: GITHUB_REPO,
+      name: version_number
+    )
+
+    pr_urls = create_backmerge_prs!
+
+    # It's possible that no backmerge was created when:
+    #
+    # - there are no hotfixes in development and the next release code freeze has not been started
+    # - nothing changes in the current release branch since release finalization
+    #
+    # As a matter of fact, in the context of Simplenote Android, the above is the most likely scenario.
+    style, message = if pr_urls.empty?
+                       ['info', 'No backmerge PR was required']
+                     else
+                       [
+                         'success', <<~MESSAGE
+                           The following backmerge PR#{pr_urls.length > 1 ? '(s) were' : ' was'} created:
+                           #{pr_urls.map { |url| "- #{url}" }}
+                         MESSAGE
+                       ]
+                     end
+    buildkite_annotate(style: style, context: 'backmerge-prs-outcome', message: message) if is_ci
+    UI.success(message)
+
+    # At this point, an intermediate branch has been created by creating a backmerge PR to a hotfix or the next version release branch.
+    # This allows us to safely delete the `release/*` branch.
+    # Note that if a hotfix or new release branches haven't been created, the backmerge PR won't be created as well.
+    delete_remote_git_branch!(current_branch)
+  end
+
   lane :trigger_beta_build do |branch_to_build:|
     trigger_buildkite_release_build(branch: branch_to_build, beta: true)
   end
@@ -335,3 +388,11 @@ def delete_old_changelogs_and_commit(version:)
     allow_nothing_to_commit: true
   )
 end
+
+# Delete a branch from the GitHub remote, after having removed any GitHub branch protection.
+#
+def delete_remote_git_branch!(branch_name, remote: 'origin')
+  remove_branch_protection(repository: GITHUB_REPO, branch: branch_name)
+
+  Git.open(Dir.pwd).push(remote, branch_name, delete: true)
+end