Release notes for all versions of PostgreSQL can be found on-line at
https://www.postgresql.org/docs/current/release.html

Distribution file sets include release notes for their version and preceding
versions.  Visit the file doc/src/sgml/html/release.html in an HTML browser.

On Debian systems, the release notes are contained in the postgresql-doc-*
packages, located in /usr/share/doc/postgresql-doc-*/html/release.html.
