### Godot version

3.5.1.stable, v4.0.beta.custom_build [0a87ead64]

### System information

Ubuntu 22.04.1

### Issue description

For example, if project has name "300spartans", we can't enter the "org.godotengine.300spartans" to "Unique Name" field. We get the error "Invalid package name: A digit cannot be the first character in a package segment". But , if we enter "org.godotengine.$genname", there will be no error message. When exporting, the name will be set as "org.godotengine.spartans" without "300" in beginning, but user will not be informed about it. I think, it is not very good. It may not be worth making changes to the source code, but mention it in the documentation.

### Steps to reproduce

See issue description.

### Minimal reproduction project

N/A
copy_args.push_back("-Pexport_path=file:" + export_path);
		copy_args.push_back("-Pexport_filename=" + export_filename);
		print_verbose("Copying Android binary using gradle command: " + String("\n") + build_command + " " + join_list(copy_args, String(" ")));
		int copy_result = EditorNode::get_singleton()->execute_and_show_output(TTR("Moving output"), build_command, copy_args);
		if (copy_result != 0) {
			add_message(EXPORT_MESSAGE_ERROR, TTR("Export"), TTR("Unable to copy and rename export file, check gradle project directory for outputs."));
			return ERR_CANT_CREATE;
		}
		print_verbose("Successfully completed Android custom build.");
		return OK;
	}
	
	print_verbose("Starting legacy build system...");
	if (p_debug) {
		src_apk = p_preset->get("custom_template/debug");
	} else {
		src_apk = p_preset->get("custom_template/release");
	}
	src_apk = src_apk.strip_edges();
	if (src_apk.is_empty()) {
		if (p_debug) {
			src_apk = find_export_template("android_debug.apk");
		} else {
			src_apk = find_export_template("android_release.apk");
		}
		if (src_apk.is_empty()) {
			add_message(EXPORT_MESSAGE_ERROR, TTR("Export"), vformat(TTR("Package not found: \"%s\"."), src_apk));
			return ERR_FILE_NOT_FOUND;
		}
	}