Update guided setup requirements in our docs (#184)

In pganalyze/collector#438 we updated host OS
requirements for the collector, but we did not update our guided setup
requirements in our instructions.

Update the docs to reflect the new requirements.

install/self_managed/00_choose_setup_method.mdx

@@ -32,7 +32,7 @@ and make necessary changes on your behalf
 The guided setup program is our recommended way to set up on systems that meet the requirements:
 
  * pganalyze-collector will be installed on the same system as your Postgres server
- * Server runs Ubuntu 14.04 or newer, or Debian 10 or newer
+ * Server runs Ubuntu 20.04 or newer, or Debian 11 or newer
  * Postgres version 10 or newer
 
 Otherwise, you'll need to follow the manual setup instructions.