Enable opt-in warning for deprecated view_component slot setter

What does this MR do and why?

One step of Upgrade to view_component v3.0 (#408988 - closed)

This enables the opt-in warn_on_deprecated_slot_setter deprecation warnings of the view_component gem in all our components.

And it fixes the warnings that were logged when running our component specs themselves: rspec spec/components.

There is still about 140 other lines in about 50 files (in app/views) that need to be updated, too. These will be done in follow-up MRs.

How to set up and validate locally

• Run bundle exec rspec spec/components and see no deprecation warnings.
• Run bundle exec rspec spec/views/admin/projects/_form.html.haml_spec.rb as an example for the remaining deprecation warnings.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.