### 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
#
from __future__ import absolute_import, division, print_function
__metaclass__ = type
DOCUMENTATION = '''
---
module: yum_repository
author: Jiri Tyr (@jtyr)
version_added: '2.1'
short_description: Add or remove YUM repositories
description:
  - Add or remove YUM repositories in RPM-based Linux distributions.
  - If you wish to update an existing repository definition use M(community.general.ini_file) instead.
options:
  async:
    description:
      - If set to C(yes) Yum will download packages and metadata from this
        repo in parallel, if possible.
    type: bool
    default: 'yes'
  bandwidth:
    description:
      - Maximum available network bandwidth in bytes/second. Used with the
        I(throttle) option.
      - If I(throttle) is a percentage and bandwidth is C(0) then bandwidth
        throttling will be disabled. If I(throttle) is expressed as a data rate
        (bytes/sec) then this option is ignored. Default is C(0) (no bandwidth