.github/workflows: adapting last v2 CI changes

[darccio]

What does this PR do?

Cleans up main CI, removing v2 stuff. Everything now lives in v2-dev, invoked from the v2-nightly.yml.

Motivation

Simplify CI removing v2 details from main.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.

For Datadog employees:

• If this PR touches code that handles credentials of any kind, such as Datadog API keys, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2023-12-21 19:52:22

Comparing candidate commit dd555d9 in PR branch dario.castane/cherrypick-v2-ci with baseline commit 82f5b7e in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 40 metrics, 1 unstable metrics.

[RomainMuller]

That makes them tremendously painful to update (since it needs to be on the branch before it can be tested); but unfortunately GitHub offers no better way...

[darccio]

I'm not fond of this too, but you said it: GitHub offers no better way. Thanks for the approval!