### 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
retries:
    description:
      - Set the number of times any attempt to retrieve a file should retry
        before returning an error. Setting this to C(0) makes yum try forever.
    type: str
    default: '10'
  s3_enabled:
    description:
      - Enables support for S3 repositories.
      - This option only works if the YUM S3 plugin is installed.
    type: bool
    default: 'no'
  skip_if_unavailable:
    description:
      - If set to C(yes) yum will continue running if this repository cannot be
        contacted for any reason. This should be set carefully as all repos are
        consulted for any given command.
    type: bool
    default: 'no'
  ssl_check_cert_permissions:
    description:
      - Whether yum should check the permissions on the paths for the
        certificates on the repository (both remote and local).
      - If we can't read any of the files then yum will force
        I(skip_if_unavailable) to be C(yes). This is most useful for non-root
        processes which use yum on repos that have client cert files which are
        readable only by root.
    type: bool
    default: 'no'
  sslcacert: