REST API: Migrate leaderboards endpoint

[itsmeichigo]

Part of #8544

Description

This PR migrates the leaderboards endpoint to enable REST API. Changes include:

• Updated LeaderboardsListMapper to parse contents without the data envelope
• Enabled REST API for the loadLeaderboards request.

Testing instructions

• In DefaultFeatureFlagService, enable the flag for applicationPasswordAuthenticationForSiteCredentialLogin and build the app.
• Log out of the app or skip onboarding if needed.
• On the prologue screen, select "Enter your site address".
• Enter your test store address and tap "Continue".
• Enter the correct site credentials for your account in the test store.
• After the login succeeds, you should be navigated to the home screen.
• In Xcode console log, notice the event 🔵 Tracked dashboard_top_performers_loaded. This means top performers have been loaded successfully.

Please feel free to test again by logging in with a WPCom account. The top performer fetch should still succeed.

Screenshots

N/A

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• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-woocommerce]

	Warnings
⚠️	This PR is assigned to a milestone which is closing in less than 2 days Please, make sure to get it merged by then or assign it to a later expiring milestone

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[wpmobilebot]

You can test the changes from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr8546-c76f163 on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[jaclync]

LGTM, just had a question about whether we support leaderboard stats WC versions under 6.7 - I tried a JN site with WC version 6.3 using the WooCommerce Beta Tester plugin, and the leaderboard API request had a 404 error.

[jaclync]

any reasons we don't allow availableAsRESTRequest: true for the deprecated leaderboards request loadLeaderboardsDeprecated below for sites with WC version under 6.7? is it because application passwords are supported for higher WC version?

[itsmeichigo]

Thanks for catching - I missed this 😅 Application password only has requirement for WordPress 5.6 minimum, so we should support WC under 6.7 I believe. Updated in c76f163.