Disco: post navigation link to lowercase (#6416)

disco/theme.json

@@ -456,7 +456,8 @@
             "core/post-navigation-link": {
                 "typography": {
                     "fontSize": "var(--wp--preset--font-size--large)",
-                    "textDecoration": "none"
+                    "textDecoration": "none",
+                    "textTransform": "lowercase"
                 },
                 "elements": {
                     "link": {