Docs: Fix README upgrade mention (#32313)

The README still mentioned Elasticsearch 1.x. This commit links to the
documentation instead and properly formats the last paragraph.

README.textile

@@ -209,10 +209,6 @@ The distribution for each project will be created under the @build/distributions
 
 See the "TESTING":TESTING.asciidoc file for more information about running the Elasticsearch test suite.
 
-h3. Upgrading from Elasticsearch 1.x?
+h3. Upgrading from older Elasticsearch versions
 
-In order to ensure a smooth upgrade process from earlier versions of
-Elasticsearch (1.x), it is required to perform a full cluster restart. Please
-see the "setup reference":
-https://www.elastic.co/guide/en/elasticsearch/reference/current/setup-upgrade.html
-for more details on the upgrade process.
+In order to ensure a smooth upgrade process from earlier versions of Elasticsearch, please see our "upgrade documentation":https://www.elastic.co/guide/en/elasticsearch/reference/current/setup-upgrade.html for more details on the upgrade process.