text-highlight: Update 'highlight' arguments

The highlight itself claims on the terminal:

   highlight: deprecated option 'failsafe' will be removed in v4.0 stable; valid alternatives: 'force'

thus replace the deprecated 'failsafe' with the new 'force' argument.

src/modules/text-highlight/e-mail-formatter-text-highlight.c

@@ -296,7 +296,7 @@ emfe_text_highlight_format (EMailFormatterExtension *extension,
 			"--include-style",
 			"--inline-css",
 			"--encoding=none",
-			"--failsafe",
+			"--force",
 			NULL };
 
 		if (!emfe_text_highlight_formatter_is_enabled ()) {