MM-14143 config cleanup final (#10374)

* TestGetLicenseFileFromDisk: avoid using fileutils.FindConfigFile

* config: abstract config-related file access, extend memory store

* simplify config validate to avoid file knowledge

* fix relative file tests

* cluster: fix ConfigChanged event

The old and new configurations were swapped when notifying the enterprise code of configuration changes, creating needless instability in propagating config updates across a cluster.

* config/database: ignore duplicates

* test cleanup

* remove unnecessary Save() in test

utils/fileutils/fileutils.go

@@ -85,15 +85,3 @@ func FindDir(dir string) (string, bool) {
 
 	return found, true
 }
-
-// FindConfigFile attempts to find an existing configuration file. fileName can be an absolute or
-// relative path or name such as "/opt/mattermost/config.json" or simply "config.json". An empty
-// string is returned if no configuration is found.
-func FindConfigFile(fileName string) (path string) {
-	found := FindFile(filepath.Join("config", fileName))
-	if found == "" {
-		found = FindPath(fileName, []string{"."}, nil)
-	}
-
-	return found
-}