<!--- Verify first that your feature was not already discussed on GitHub -->
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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 -->

if user != "root" and "PROXY" not in priv and not append_privs:
                        msg = "Privileges updated"
                        if module.check_mode:
                            return (True, msg)
                        privileges_revoke(cursor, user, host, db_table, priv, grant_option)
                        changed = True
            
            
            for db_table, priv in iteritems(new_priv):
                if db_table not in curr_priv:
                    msg = "New privileges granted"
                    if module.check_mode:
                        return (True, msg)
                    privileges_grant(cursor, user, host, db_table, priv)
                    changed = True
            
            
            db_table_intersect = set(new_priv.keys()) & set(curr_priv.keys())
            for db_table in db_table_intersect:
                priv_diff = set(new_priv[db_table]) ^ set(curr_priv[db_table])
                if len(priv_diff) > 0:
                    msg = "Privileges updated"
                    if module.check_mode:
                        return (True, msg)
                    if not append_privs:
                        privileges_revoke(cursor, user, host, db_table, curr_priv[db_table], grant_option)
                    privileges_grant(cursor, user, host, db_table, new_priv[db_table])
                    changed = True
    return (changed, msg)
def user_delete(cursor, user, host, host_all, check_mode):