DRY new_project_sast_enabled experiment field with partials

What does this MR do and why?

This MR encapsulations some duplication between experiment variations in a partial.

(Copied from !76899 (comment 784278341)):

• DRY'ing this removes any concern about error between the consistency of the experiments
• It's not clear at first glance what the complete and actual difference is between the experiments (even though the names are descriptive).
• Moving this to a partial improves it's own readability (because it doesn't have to be nested) + the readability of the original file since it doesn't include a big conditional block.

I'd rather have a temporary partial than temporary duplication.

Screenshots

Nothing should have changed in this MR and here's screenshots verifying locally:

control	cadidate	free_indicator	unchecked_cadidate	unchecked_free_indicator

How to set up and validate locally

(Copied from !76899 (merged))

1. Enable the experiment

   Feature.enable(:new_project_sast_enabled)

2. Assign the variants you want to, for your test user (assuming root is your test user)

   include Gitlab::Experiment::Dsl
   experiment(:new_project_sast_enabled, :control, user: User.first) # no checkboxes should be visible
   experiment(:new_project_sast_enabled, :candidate, user: User.first) # checkbox should be visible and checked
   experiment(:new_project_sast_enabled, :free_indicator, user: User.first) # checkbox should be visible and checked, and you should have a label indicating that it's free if you check it
   experiment(:new_project_sast_enabled, :unchecked_candidate, user: User.first) # checkbox should be visible and unchecked
   experiment(:new_project_sast_enabled, :unchecked_free_indicator, user: User.first) # checkbox should be visible and unchecked, with the free label

3. Create a new blank project.

MR acceptance checklist

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