Doc: fix mention of pg_dump's minimum supported server version.

runtime.sgml contains a passing reference to the minimum server
version that pg_dump[all] can dump from.  That was 7.0 for many
years, but when 64f3524e2 raised it to 8.0, we missed updating this
bit.  Then when 30e7c175b raised it to 9.2, we missed it again.

Given that track record, I'm not too hopeful that we'll remember
to fix this in future changes ... but for now, make the docs match
reality in each branch.

Noted by Daniel Westermann.

Discussion: https://postgr.es/m/GV0P278MB041917EB3E2FE8704B5AE2C6D2DC9@GV0P278MB0419.CHEP278.PROD.OUTLOOK.COM

doc/src/sgml/runtime.sgml

@@ -1762,7 +1762,7 @@ $ <userinput>kill -INT `head -1 /usr/local/pgsql/data/postmaster.pid`</userinput
     version of
     <productname>PostgreSQL</productname>, to take advantage of enhancements
     that might have been made in these programs.  Current releases of the
-    dump programs can read data from any server version back to 7.0.
+    dump programs can read data from any server version back to 8.0.
    </para>
 
    <para>