[WPB-10324] Drop legacy notification endpoints

[mdimjasevic]

Tracked by https://wearezeta.atlassian.net/browse/WPB-10324.

Checklist

• Add a new entry in an appropriate subdirectory of changelog.d
• Read and follow the PR guidelines

[fisx]

did you also create the confluence page and add it there? seems like a good idea to me, especially since we might all completely forget by the time this becomes relevant.

[mdimjasevic]

I haven't yet, as I am still not 100% sure what version this should be reflected in. The latest discussion in the ticket suggests this won't be done in version 9, but in a much later version and perhaps this PR can be closed, but let's wait and see how the discussion evolves.

[fisx]

Suggested change

	-- it is however theoretically possible to have a different number of
	-- it is however theoretically (and practically) possible to have a different number of

[fisx]

this will conflict with #4356

but maybe that's fine?

[mdimjasevic]

Yes, that's totally fine. I am aware of this. There's no way to avoid conflicts if we work in parallel on related stuff. I see the PR has been merged in the PR so I'll rebase this PR to resolve the conflicts.

[akshaymankar]

Let's merge it when the work for clients is scheduled, so we avoid creating chores to keep updating this version number.

[CLAassistant]

All committers have signed the CLA.