Deprecate Blackfriday and fix a potential deadlock in config

Note that the deadlock has not been seen earlier, in tests on in real Hugo sites.

Fixes #8792
Fixes #8791

hugolib/config.go

@@ -107,12 +107,18 @@ func LoadConfig(d ConfigSourceDescriptor, doWithConfig ...func(cfg config.Provid
 		}
 	}
 
+	// Config deprecations.
 	// We made this a Glob pattern in Hugo 0.75, we don't need both.
 	if l.cfg.GetBool("ignoreVendor") {
 		helpers.Deprecated("--ignoreVendor", "--ignoreVendorPaths **", true)
 		l.cfg.Set("ignoreVendorPaths", "**")
 	}
 
+	if l.cfg.GetString("markup.defaultMarkdownHandler") == "blackfriday" {
+		helpers.Deprecated("markup.defaultMarkdownHandler=blackfriday", "See https://gohugo.io//content-management/formats/#list-of-content-formats", false)
+
+	}
+
 	// Some settings are used before we're done collecting all settings,
 	// so apply OS environment both before and after.
 	if err := l.applyOsEnvOverrides(d.Environ); err != nil {