resources/page: Deprecate cascade._target in favor of cascade.target

Closes #14341

hugolib/config.go

@@ -119,7 +119,7 @@ posts = '/posts/:year/:month/:title/'
 category = 'categories'
 series = 'series'
 tag = 'tags'
-  
+
 [module]
 [module.hugoVersion]
 min = '0.80.0'
@@ -138,12 +138,12 @@ precision = 2
 
 [[cascade]]
 background = 'yosemite.jpg'
-[cascade._target]
+[cascade.target]
   kind = 'page'
   path = '/blog/**'
 [[cascade]]
 background = 'goldenbridge.jpg'
-[cascade._target]
+[cascade.target]
   kind = 'section'
 
 

resources/page/page_matcher.go

@@ -199,8 +199,12 @@ func (d cascadeConfigDecoder) mapToPageMatcherParamsConfig(m map[string]any) (Pa
 	}
 
 	for k, v := range m {
-		switch strings.ToLower(k) {
+		klc := strings.ToLower(k)
+		switch klc {
 		case "_target", "target":
+			if klc == "_target" {
+				hugo.Deprecate("cascade._target", "Use cascade.target instead, see https://gohugo.io/content-management/front-matter/#target", "v0.156.0")
+			}
 			var target PageMatcher
 			if err := d.decodePageMatcher(v, &target); err != nil {
 				return pcfg, err