What to do with comments...
I don't know if there's a better approach available than just manually removing all the comments that link to the old domain (and have thus become spam) and replacing them, in cases where it makes sense to do so, with new comments.
As I mentioned, moderators can delete comments, but I don't know if they have any automated means to make this change.1
I am hoping a moderator will weigh in on this question.
We don't have any automated mechanisms for this. --Thomas W.
1 Meta.SE has Is there a way to fix broken links in comments? Can the mods edit comments? but that question is over five years old, very general, and doesn't address the case of numerous comments breaking in one specific way due to an unusual event of a one single highly useful domain having been subverted in a way its former owner cannot control, possibly for the purpose of generating traffic from our site.
Your site and blog are important, useful resources for the Ubuntu community, and your old domain appears to have been bought to capture unintended traffic. It's good this is being fixed. Thank you for your actions thus far!
As I write this, I see you are already fixing the posts. I would be pleased to make some of these edits, but I don't want to step on your toes. Accidental simultaneous edits to the same post can be confusing and annoying. I recall there is a limit on how many edits a user can make to their own posts in a day, so if you reach that--or for any other reason would benefit from help--please let me know and I'll make some of the edits. (Of course, that limit applies just to your own posts, not posts by other users that link to the domain.)
Sometimes people are concerned about how a large number of edits carried out over a short time can push other posts off the front page. But this is a case where I don't think we need to worry about edit volume:
The total number of edits is just not that high. It was well less than a hundred even as I began to write this. (Really this is enough of a reason not to worry, by itself.)
The spammy nature of the site--identifiable by examining the site and the links therein, or by checking its whois record and noticing that the current contact person has been associated with questionable prescription-drug related sites--probably justifies any volume of edits.
Furthermore, I think the main purpose served by our general reluctance to make a huge volume of edits in a short time is that it has the effect of putting off edits that might not need to happen. These edits do need to happen. Every post and edit (except some performed automatically by the system) bumps the affected question on the front page, and usually something drops off the front page as a result.
For a fixed number of edits that are going to be done anyway, it seems to me that doing this fast rather than slow is not really a problem. It even increases the chance that the post dropped from the front page will be one of the ones brought there by one of your edits.
Finding posts and comments containing the old domain:
Michelle's post search and comment SEDE query should be sufficient for this. I've added these alternatives in case they are of interest, but I'm not suggesting they should be preferred.
This SEDE query finds posts that contain
bodhizazen.net in the body. It returns 100 hits. Although it returns more hits than searching on the site with
url as Michelle suggests, I would not assume it is better. The Stack Exchange Data Explorer searches snapshots of the site (updated weekly as Michelle mentions), so this query is probably just turning up posts in which the URL has already been edited.
This SEDE query finds posts that have comments that contain
bodhizazen.net. It differs from Michelle's query in that it lists posts and gives post links, rather than comment links. Like that query is currently shows 85 results (which is what one would expect unless there are posts with more that one comment with the old domain). As far as I know, either query is sufficient.