Update Debian base image to bullseye

What does this MR do?

Update the base image of the Debian images from debian:buster-slim to debian:bullseye-slim, as Bullseye is now the Stable release of Debian. This has added advantage of update libraries for some of our components (such as libcurl) being available directly from the distribution repositories.

Related issues

Related to gitlab-org/charts/gitlab#3080 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated