Chore: Remove public vars in setting package (#81018)

Removes the public variable setting.SecretKey plus some other ones. 
Introduces some new functions for creating setting.Cfg.

pkg/services/rendering/rendering.go

@@ -244,7 +244,7 @@ func (rs *RenderingService) renderUnavailableImage() *RenderResult {
 	imgPath := "public/img/rendering_plugin_not_installed.png"
 
 	return &RenderResult{
-		FilePath: filepath.Join(setting.HomePath, imgPath),
+		FilePath: filepath.Join(rs.Cfg.HomePath, imgPath),
 	}
 }