infra: remove generate contrib workflows from core

[emdneto]

This PR basically shifts from depending on generating workflows every time we change something in contrib to looking to contrib@main and reusing the workflow core_contrib_test.yml. With that, we only need to update contrib workflows, and there's no need to sync workflows in core anymore.

Related PR in contrib: open-telemetry/opentelemetry-python-contrib#3035

Part of #4263

[emdneto]

Note: this is the first step of a major change we want to do that is related to have generate_workflows_lib for each repo instead of core depends on generate_workflos_lib hosted in contrib

[xrmx]

So in order to get this in it requires to update our branch protecting rules that are referencing the old jobs.