fix(memberships): use gate ID for metering meta key

[dkoo]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

While testing for #4315, I discovered a bug that causes sites to store and read metering data from the wrong meta key if using Woo Memberships for content gating. This would only affects sites that have previously enabled the experimental content gating UI already, so it shouldn't affect any sites in production.

How to test the changes in this Pull Request:

1. Make sure NEWSPACK_CONTENT_GATES is not defined in wp-config.php.
2. With Woo Memberships enabled and with a plan that restricts content by active subscription, publish a content gate with metering enabled for anonymous and logged-in readers.
3. As a reader, confirm that metering works as expected while both not logged in and logged in.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[matticbot]

🎉 This PR is included in version 6.25.1 🎉

The release is available on GitHub release

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