Generate release notes in release pipeline

This commit adds new tasks to the release pipeline, generating release
notes automatically using the issues in the current milestone and
pushing that as the content of the GitHub release.

Closes gh-25922

ci/scripts/generate-changelog.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+set -e
+
+CONFIG_DIR=git-repo/ci/config
+version=$( cat version/version )
+
+java -jar /github-changelog-generator.jar \
+  --spring.config.location=${CONFIG_DIR}/changelog-generator.yml \
+  ${version} generated-changelog/changelog.md
+
+echo ${version} > generated-changelog/version
+echo v${version} > generated-changelog/tag