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

config/defaultConfigProvider.go

@@ -89,6 +89,7 @@ func (c *defaultConfigProvider) Get(k string) interface{} {
 	c.mu.RLock()
 	key, m := c.getNestedKeyAndMap(strings.ToLower(k), false)
 	if m == nil {
+		c.mu.RUnlock()
 		return nil
 	}
 	v := m[key]

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 {