chore : Deprecating FeatureToggles.IsEnabled (#113062)

* Deprecating features.IsEnabled

* add one more nolint

* add one more nolint

* Give better hints to devs in the deprecation message of IsEnabledGlobally

* adding more doc strings

* fix linter after rebase

* Extend deprecation message

pkg/api/pluginproxy/pluginproxy.go

@@ -81,6 +81,7 @@ func (proxy *PluginProxy) HandleRequest() {
 			hasSlash := strings.HasSuffix(proxy.proxyPath, "/")
 			proxy.proxyPath = path
 
+			//nolint:staticcheck // not yet migrated to OpenFeature
 			if hasSlash && !strings.HasSuffix(path, "/") && proxy.features.IsEnabled(proxy.ctx.Req.Context(), featuremgmt.FlagPluginProxyPreserveTrailingSlash) {
 				proxy.proxyPath += "/"
 			}