OSDOCS#13561: Update ocp-nminus1 and ocp-nplus1 attributes

Author: michaelryanpeter

_attributes/common-attributes.adoc

@@ -11,8 +11,8 @@
 :imagesdir: images
 :prewrap!:
 // n-1 and n+1 OCP versions relative to the current branch's {product-version} attr
-:ocp-nminus1: 4.17
-:ocp-nplus1: 4.19
+:ocp-nminus1: 4.18
+:ocp-nplus1: 4.20
 // Operating system attributes
 :op-system-first: Red{nbsp}Hat Enterprise Linux CoreOS (RHCOS)
 :op-system: RHCOS