### 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
throttling).
    type: str
    default: '0'
  baseurl:
    description:
      - URL to the directory where the yum repository's 'repodata' directory
        lives.
      - It can also be a list of multiple URLs.
      - This, the I(metalink) or I(mirrorlist) parameters are required if I(state) is set to
        C(present).
    type: list
    elements: str
  cost:
    description:
      - Relative cost of accessing this repository. Useful for weighing one
        repo's packages as greater/less than any other.
    type: str
    default: '1000'
  deltarpm_metadata_percentage:
    description:
      - When the relative size of deltarpm metadata vs pkgs is larger than
        this, deltarpm metadata is not downloaded from the repo. Note that you
        can give values over C(100), so C(200) means that the metadata is
        required to be half the size of the packages. Use C(0) to turn off
        this check, and always download metadata.
    type: str
    default: '100'
  deltarpm_percentage:
    description:
      - When the relative size of delta vs pkg is larger than this, delta is