ci: send Slack notification on nightly failures + modernize workflows

[anakin87]

I initially worked on this as part of https://github.com/deepset-ai/haystack-private/issues/256: Instead of sending DD events for CI failures and then Slack notifications from DD monitors, skip DD.

For more details, see the similar PR deepset-ai/haystack#10729.

Then I realized that these workflows can be modernized and improved, so I did the following:

• merge nightly and run_tutorials that were doing the same (running tutorials), but the first on scheduled nightly runs and the second on PRs
• use Ubuntu slim (cheaper) for simple jobs (computing test matrices)
• use newer versions of GitHub actions
• use uv for installations and stop using containers (uv pip install haystack-ai is fast)
• stop installing system dependencies: they are no longer needed; they probably were in the past, partly based on v1 tutorials, partly because Haystack container comes with limited system libraries compared to Ubuntu GitHub runners

I tested this on this PR, by triggering temporary changes. Works correctly.

In addition, installation times are impressively reduced. An example for tutorial 27: 32s vs 140s!

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[kacperlukawski]

Nice work! Left just a minor comment, but it's good to merge. Btw, does that mean we are also going to save some money due to deduplication?

[kacperlukawski]

Suggested change

	workflow_dispatch: # Activate this workflow manually
	workflow_dispatch:

Isn't the comment outdated?

[anakin87]

this is a just a comment that informs that using this event, you can activate the workflow manually
More info: https://docs.github.com/en/actions/reference/workflows-and-actions/events-that-trigger-workflows#workflow_dispatch

Currently, in https://github.com/deepset-ai/haystack-tutorials/actions/workflows/nightly.yml there is this button