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##### SUMMARY
<!--- Describe the new feature/improvement briefly below -->
New Percona/MySQL 8 privileges with underscore is not working. 
the following error is coming 

`MSG: invalid privileges string: Invalid privileges specified: frozenset({'BACKUP_ADMIN'})`

##### ISSUE TYPE
- Feature Idea

##### COMPONENT NAME
<!--- Write the short name of the module, plugin, task or feature below, use your best guess if unsure -->
mysql_user module
most likely the new privileges needs to be added to lib/ansible/modules/database/mysql/mysql_user.py

##### ADDITIONAL INFORMATION
<!--- Describe how the feature would be used, why it is needed and what it would solve -->

<!--- Paste example playbooks or commands between quotes below -->
```yaml

```

<!--- HINT: You can also paste gist.github.com links for larger files -->

cursor = None
    try:
        if check_implicit_admin:
            try:
                cursor, db_conn = mysql_connect(module, 'root', '', config_file, ssl_cert, ssl_key, ssl_ca, db,
                                                connect_timeout=connect_timeout)
            except Exception:
                pass
        if not cursor:
            cursor, db_conn = mysql_connect(module, login_user, login_password, config_file, ssl_cert, ssl_key, ssl_ca, db,
                                            connect_timeout=connect_timeout)
    except Exception as e:
        module.fail_json(msg="unable to connect to database, check login_user and login_password are correct or %s has the credentials. "
                             "Exception message: %s" % (config_file, to_native(e)))
    if not sql_log_bin:
        cursor.execute("SET SQL_LOG_BIN=0;")
    if priv is not None:
        try:
            mode = get_mode(cursor)
        except Exception as e:
            module.fail_json(msg=to_native(e))
        try:
            priv = privileges_unpack(priv, mode)
        except Exception as e:
            module.fail_json(msg="invalid privileges string: %s" % to_native(e))
    if state == "present":
        if user_exists(cursor, user, host, host_all):
            try:
                if update_password == 'always':
                    changed, msg = user_mod(cursor, user, host, host_all, password, encrypted,