Upgrade stripe.js from v3 to clover

[coderabbitai]

Consider upgrading from stripe.js v3 to the latest "clover" version as mentioned in the Stripe documentation.

Context:
This was identified during the review of PR #137 which adds Stripe subscription checkout endpoint.

References:

• PR: Add server-side Stripe checkout for recurring donations #137
• Stripe versioning documentation: https://docs.stripe.com/sdks/stripejs-versioning
• Requested by: @tobihagemann