How to perform a safe upgrade and rollback of a Bitbucket instance

What are the recommended steps to upgrade a Bitbucket Server or Data Center deployment while ensuring a reliable rollback path if issues arise? Please outline the preparation, backup, upgrade execution, verification, and rollback procedures, highlighting any assumptions about environment (e.g., supported versions, database compatibility) and noting where official documentation should be consulted. Include steps for testing the upgrade in a staging environment, reviewing release notes, validating add‑on compatibility, and documenting custom configurations before proceeding.