Reliably fetch logs for imagebuild and prebuild itself

[geropl]

Description

This PR deprecates an older mechanism to stream image build logs. Removing that revealed that it was shadowing reliabilty problems, so those got fixed in the frontend as well. 🙈

I hid the changes behind a feature flag to safely test the effects in staging + prod (different ingress infrastructure).

Related Issue(s)

Fixes #11024

How to test

• create a team and project (e.g. on this test repo)
• push a commit, for instance to this branch
• switch to "Branches", wait for the prebuild to start and click on the Prebuild Details view
• open a second tab on "Branches", and once the prebuild is there, click "start workspace" right next to it
• observe that a) streaming of image build logs and b) streaming of prebuild logs works as expected
• go to ConfigCat, and add a filter for your project to include the projectId and set the flag deprecateOldImageLogsMechanism to true
• repeat tests above ☝️

Release Notes

Improve reliability of log streaming for image builds and prebuilds

Documentation

Werft options:

• /werft with-preview

[geropl]

@jldec I just tested, and it worked nicely for me. 👍 Do you want to have a look?

Note: the status message does not fit. We can improve that in a follow-up.

[jldec]

LGTM
Image bulld logs appear properly for both prebuild-detail and start-workspace views.

(I was unable to figure out how to test the code path where the feature flag determines whether to show an error or use the old mechanism)

/hold in case you want to clarify or do more technical review

[geropl]

(I was unable to figure out how to test the code path where the feature flag determines whether to show an error or use the old mechanism)

Sorry, should have explained 🙇 But I think it's fine as is, already did that twice yesterday.
Thank you! 🙏

/unhold