ci: disable EPEL-7 tests

[kdudka]

They started to fail because EPEL-7 is EOL:

[08:31:04] [E] [worker_0] [artemis] [597190e3-c5dc-4b67-9ea7-6803ab0302cf] [pre-artifact-installation] Last 30 lines of Ansible stdout:
---v---v---v---v---v---
    "skip_reason": "Conditional result was False"
}

TASK [Enable Powertools repository (CentOS 8, CentOS Stream 8)] ****************
skipping: [3.136.112.131] => {
    "changed": false,
    "skip_reason": "Conditional result was False"
}

TASK [include_tasks] ***********************************************************
included: /CONFIG/guest-setup/pre-artifact-installation/install-epel.yaml for 3.136.112.131

TASK [Install EPEL on CentOS < 9 and CentOS Stream < 9 (except CentOS Stream 9)] ***
FAILED - RETRYING: [3.136.112.131]: Install EPEL on CentOS < 9 and CentOS Stream < 9 (except CentOS Stream 9) (5 retries left).
FAILED - RETRYING: [3.136.112.131]: Install EPEL on CentOS < 9 and CentOS Stream < 9 (except CentOS Stream 9) (4 retries left).
FAILED - RETRYING: [3.136.112.131]: Install EPEL on CentOS < 9 and CentOS Stream < 9 (except CentOS Stream 9) (3 retries left).
FAILED - RETRYING: [3.136.112.131]: Install EPEL on CentOS < 9 and CentOS Stream < 9 (except CentOS Stream 9) (2 retries left).
FAILED - RETRYING: [3.136.112.131]: Install EPEL on CentOS < 9 and CentOS Stream < 9 (except CentOS Stream 9) (1 retries left).
fatal: [3.136.112.131]: FAILED! => {
    "attempts": 5,
    "changed": false
}

MSG:

Failure talking to yum: Cannot find a valid baseurl for repo: base/7/x86_64

PLAY RECAP *********************************************************************
3.136.112.131              : ok=15   changed=2    unreachable=0    failed=1    skipped=22   rescued=0    ignored=3

---^---^---^---^---^---
[08:31:04] [E] [CentOS-7:x86_64:/plans/ci] guest setup failed: Failure during Ansible playbook execution

[kdudka]

@lzaoral Thanks for review!