Update dmwm-base and wmagent-base images tag to 20240912; adopt it across WM images

[amaltaro]

Fixes dmwm/WMCore#11997

With this PR, the following changes are provided:

1. update the default manage script in the dmwm-base image (see Ensure X509_USER_PROXY is set for MSUnmerged restarts #1532)
2. pull the latest python3.8-bullseye image (change of python version from 3.8.16 to 3.8.20
3. stop importing into our images the now obsolete /etc/vomses from the old cmsweb-base image.

In addition, I compared the old (20230525) and the new (20230912) dmwm-base images and they come with the same kernel version. Lastly, CERN Harbor reports about 1k less vulnerabilities for wmagent-base, also compared against the previous one.

[amaltaro]

BTW, the relevant dmwm-base and wmagent-base images have been uploaded and tagged as pypi-20240912.
The remaining WM images will be built once we cut a new WMCore tag.

[vkuznet]

Alan, I want to remind about different lifetime policies for images, the regular images keep about 1-3 months, while stable images much longer. I suggest for all images to be used in production to use stable suffix. So I suggest to make dmwm-base and wmagent-base to use tag pypi-20240912-stable.

[amaltaro]

@vkuznet thanks for the reminder. I have just uploaded the relevant pypi-20240912-stable tags for dmwm-base and wmagent-base images. It also helped me spot a mistake in the previous tag, which said "2023" instead of "2024" (!)

Please let me know if you see anything else pending. Thanks!