Modify UX for dependency proxy project settings form

What does this MR do and why?

Follow-up to Adds support for updating maven dependency prox... (!139535 - merged) based on discussion.

Refactor the way dependency proxy form is updated on the frontend based on 1.) of above mentioned discussion

• Allow updating toggle & maven fields using a single form.
• Remove apollo cache update related code required for immediate update of toggle. This was added in Calls mutation when dependency proxy packages s... (!139379 - merged)
• Adds feature tests which should pass when bug fix dependency is merged.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

before	after
Screen_Recording_2023-12-13_at_1.08.31_am	Screen_Recording_2024-01-02_at_10.47.15_pm

How to set up and validate locally

The dependency proxy for packages has a few requirements:

1. have packages -> enabled set to true in gitlab.yml.

2. have dependency_proxy -> enabled set to true in gitlab.yml.

3. have the packages feature enabled in the project's settings. Settings -> General -> Visiblity, project features, permissions -> Package registry (checkbox enabled.)

4. have a GitLab license. Premium or more.

5. have the related feature flag turned on:

   Feature.enable(:packages_dependency_proxy_maven)

6. Visit Project > Settings > Packages and Registries

7. Under the Dependency Proxy settings block, try updating the form

8. Ensure the following:

   1. Toggle gets updated immediately
   2. Submit button is visible & gets disabled when submitting.
   3. Submitting the form shows a toast when successful.
   4. Submitting the form shows an alert with correct error message when request fails.

Feature test, should pass when bug fix is merged

bundle exec rspec ee/spec/features/projects/settings/packages_spec.rb

Related to #410726 (closed)