Ensure release notes from older version still load without 404 errors.

Ensure release notes from older version still load without 404 errors.

An upcoming change in PostgreSQL keeps only release notes in their
current versions. So e.g., PostgreSQL 11 only has the release notes
for 11.0, 11.1, 11.2 etc.

Thus, for PostgreSQL 9.4 on up, any URLs such as /docs/10/release-9-6-8.html
would return a 404. Additonal, URLs with /current/ and /devel/ would break
as well.

This patch prevents these 404s from occuring by redirecting users accessing
release notes in the 9.4 and above documentation to the older versions
of the release notes by redirecting them to the notes in the specific
version. So in the above example, a user would be redirect to
/docs/9.6/release-9-6-8.html

As we do not have documentation on the website past 7.2, any release
notes from an older version are redirected to the release notes tree
in 7.2