“Howdy” is in a translatable string, so you could use a gettext function in your functions.php to translate it to something else.
$ grep -i -r howdy * |grep -v plugins
wordpress/wp-includes/ms-functions.php: $welcome_email = __( 'Howdy USERNAME,
wordpress/wp-includes/ms-functions.php: $text = __( 'Howdy USERNAME,
wordpress/wp-includes/ms-functions.php: $email_text = __( 'Howdy ###USERNAME###,
wordpress/wp-includes/admin-bar.php: $howdy = sprintf( __( 'Howdy, %s' ), '<span class="display-name">' . $current_user->display_name . '</span>' );
wordpress/wp-includes/admin-bar.php: 'title' => $howdy . $avatar,
wordpress/wp-includes/user.php: $email_text = __( 'Howdy ###USERNAME###,
wordpress/wp-includes/user.php: 'Howdy,
wordpress/wp-includes/user.php: 'Howdy,
wordpress/wp-includes/user.php: 'Howdy,
wordpress/wp-includes/user.php: 'Howdy,
wordpress/wp-admin/js/user-profile.js: * Replaces "Howdy, *" in the admin toolbar whenever the display name dropdown is updated for one's own profile.
wordpress/wp-admin/includes/schema.php: $welcome_email = __( 'Howdy USERNAME,
wordpress/wp-admin/includes/class-wp-automatic-updater.php: $body .= sprintf( __( 'Howdy! Your site at %1$s has been updated automatically to WordPress %2$s.' ), home_url(), $core_update->current );
wordpress/wp-admin/includes/misc.php: $email_text = __( 'Howdy ###USERNAME###,
wordpress/wp-admin/includes/file.php:'Howdy,
wordpress/wp-admin/install.php:<h1><?php _ex( 'Welcome', 'Howdy' ); ?></h1>
wordpress/wp-admin/install.php:<h1><?php _ex( 'Welcome', 'Howdy' ); ?></h1>
wordpress/wp-admin/ms-delete-site.php: $content = __( "Howdy ###USERNAME###,
https://codex.www.ads-software.com/Plugin_API/Filter_Reference/gettext