### Summary

Adding a yum repository to a centos/rhel 8 system with yum_repository module adds async option (deprecated) to repository-file.

Inspecting /var/log/dnf.log on centos/rhel 8 after running `dnf repolist` produces the following errors.
```
2021-07-30T10:38:19+0200 DEBUG Unknown configuration option: async = 0 in /etc/yum.repos.d/epel.repo
```
Seems yum_repository module should omit the async option when target system is centos/rhel8?

best regards, Mikael

### Issue Type

Bug Report

### Component Name

yum_repository

### Ansible Version

```console
$ ansible --version
  config file = None
  configured module search path = ['/home/$USER/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /usr/local/lib/python3.9/site-packages/ansible
  ansible collection location = /ansible-collections:/usr/share/ansible/collections
  executable location = /usr/local/bin/ansible
  python version = 3.9.5 (default, Jun 23 2021, 15:10:59) [GCC 8.3.0]
  jinja version = 3.0.1
  libyaml = True
```


### Configuration

```console
$ ansible-config dump --only-changed
none
```


### OS / Environment

CentOS Linux release 8.4.2105

### Steps to Reproduce

<!--- Paste example playbooks or commands between quotes below -->
```yaml (paste below)
---
- hosts: all
  become: true
  tasks:
  - name: Add repository
    yum_repository:
      name: epel
      description: EPEL YUM repo
      baseurl: https://download.fedoraproject.org/pub/epel/$releasever/$basearch/
      gpgcheck: no
```


### Expected Results

```
TASK [Add repository] ***********************************************************
--- before: /etc/yum.repos.d/epel.repo
+++ after: /etc/yum.repos.d/epel.repo
@@ -0,0 +1,6 @@
+[epel]
+baseurl = https://download.fedoraproject.org/pub/epel/$releasever/$basearch/
+gpgcheck = 0
+name = EPEL YUM repo
+

changed: [centos8-host]
```
### Actual Results

```console
TASK [Add repository] ***********************************************************
--- before: /etc/yum.repos.d/epel.repo
+++ after: /etc/yum.repos.d/epel.repo
@@ -0,0 +1,6 @@
+[epel]
+async = 1
+baseurl = https://download.fedoraproject.org/pub/epel/$releasever/$basearch/
+gpgcheck = 0
+name = EPEL YUM repo
+

changed: [centos8-host]
```


### Code of Conduct

- [X] I agree to follow the Ansible Code of Conduct
if self.repofile.has_section(self.section):
            self.repofile.remove_section(self.section)
        
        self.repofile.add_section(self.section)
        
        
        
        req_params = (self.params['baseurl'], self.params['metalink'], self.params['mirrorlist'])
        if req_params == (None, None, None):
            self.module.fail_json(
                msg="Parameter 'baseurl', 'metalink' or 'mirrorlist' is required for "
                    "adding a new repo.")
        
        for key, value in sorted(self.params.items()):
            if key in self.list_params and isinstance(value, list):
                
                value = ' '.join(value)
            elif isinstance(value, bool):
                
                value = int(value)
            
            if value is not None and key in self.allowed_params:
                self.repofile.set(self.section, key, value)
    def save(self):