Refuse upgrades from pre-9.0 clusters

Commit 695b4a113a added a dependency on retrieving oldestxid from
pg_control, which only exists in 9.0 and onwards, but the check for
8.4 as the oldest version was retained. Since there has been few if
any complaints of 8.4 upgrades not working, fix by setting 9.0 as
the oldest version supported rather than resurrecting 8.4 support.

Backpatch to all supported versions.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/1973418.1657040382@sss.pgh.pa.us
Backpatch-through: v12

doc/src/sgml/ref/pgupgrade.sgml

@@ -68,7 +68,7 @@ PostgreSQL documentation
  </para>
 
   <para>
-   pg_upgrade supports upgrades from 8.4.X and later to the current
+   pg_upgrade supports upgrades from 9.0.X and later to the current
    major release of <productname>PostgreSQL</productname>, including snapshot and beta releases.
   </para>
  </refsect1>

src/bin/pg_upgrade/check.c

@@ -280,8 +280,13 @@ check_cluster_versions(void)
 	 * upgrades
 	 */
 
-	if (GET_MAJOR_VERSION(old_cluster.major_version) < 804)
-		pg_fatal("This utility can only upgrade from PostgreSQL version 8.4 and later.\n");
+	/*
+	 * The minimum version supported when this code shipped in a major version
+	 * was 8.4. This has since been raised to 9.0, but the support code for
+	 * dealing with 8.4 remains to avoid refactoring in a backbranch.
+	 */
+	if (GET_MAJOR_VERSION(old_cluster.major_version) < 900)
+		pg_fatal("This utility can only upgrade from PostgreSQL version 9.0 and later.\n");
 
 	/* Only current PG version is supported as a target */
 	if (GET_MAJOR_VERSION(new_cluster.major_version) != GET_MAJOR_VERSION(PG_VERSION_NUM))