Updated approval settings to use local form data to stop mutating outside the store

What does this MR do?

This MR fixes the Vuex usage in the group-level approval settings form. It currently mutates the store from the outside by making use of v-modal which goes against the Vuex guidelines.

The solution decided upon is to store the forms state within the component itself and then pass the local state to the store when the form is submitted. This prevents potentially invalid data from being submitted to the store early and allows us to mutate the store through actions.

Setup & Testing

1. Enable the feature flag echo "Feature.enable(:group_merge_request_approval_settings_feature_flag)" | rails c
2. View the Merge request approvals settings at [GDK_HOST]/groups/[GROUP]/-/edit
3. Toggle the checkboxes and refresh the page to see if the settings persist

Screenshots (strongly suggested)

(No visual changes)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this is behind the :group_merge_request_approval_settings_feature_flag feature flag.
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related #326510 (closed)