Add granular token enforcement to top-level group settings (FE)

What does this MR do and why?

• Add granular token enforcement to top-level group settings (frontend-only). The backend was already added in !229817 (merged)
• Render the enforcement checkbox and date picker on the top-level group's Settings > General page when feature-flag granular_personal_access_tokens_enforcement_saas is enabled.

Screenshots or screen recordings

How to set up and validate locally

1. Enable the feature flag:

Feature.enable(:granular_personal_access_tokens_enforcement)
Feature.enable(:granular_personal_access_tokens_enforcement_saas)

2. Log-in as a user and navigate to any top-level group that you are an owner of.
3. Go to Settings > General > Permissions and group features
4. Check the box and select a date under Fine-grained personal access tokens

To validate feature end-to-end:

1. Navigate to /-/user_settings/personal_access_tokens and create two tokens -> a fine-grained token with Member: Read permission on All groups and projects that I'm a member of and a Legacy token.
2. If the enforcement is disabled, you will be able to access the endpoint /api/v4/groups/<GROUP_ID>/members with both tokens. If enforcement is enabled, only fine-grained token will be able to access the endpoint.

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Related to #593961 (closed)