yum repo centos7.9.2009 is unavailable

[YU CHENGDE]

Brief Description
-----------------
After executing download_mirror.sh, it will run command "sudo -E yum -c /localdisk/loadbuild/stx/distro/tmp/yum/yum.conf --releasever=7 makecache".
However, centos 7.9.2009 repo is unavailable.

Severity
--------
Minor

Steps to Reproduce
------------------
follow the layered build document
https://docs.starlingx.io/developer_resources/layered_build_guide.html
bash download_mirror.sh

Expected Behavior
------------------
Make cache on yum repo and download dependences successful

Actual Behavior
----------------
Make cache on yum repo and download dependences failed

Reproducibility
---------------
Always

Timestamp/Logs
--------------
http://mirror.centos.org/centos/7.9.2009/atomic/x86_64/repodata/repomd.xml: [Errno 14] HTTP Error 503 - Service Unavailable
Trying other mirror.