[Link] StripePaymentSheet depends on StripeLinkCore

[bmelts-stripe]

Summary

Added StripeLinkCore as a dependency of StripePaymentSheet, and updated documentation. cc @kgaidis-stripe

Motivation

Part of the ongoing module creation process (see: #2320, #2335).

Testing

Ran through wiki instructions, everything works as expected.

Notes

I copied the wording in the documentation updates from the wiki, but I'm open to discussion on if there are better ways we should communicate this. I've also left the version undefined for MIGRATING.md - we should decide on what kind of bump we want to do for this (major or minor).

[kgaidis-stripe]

@bmelts-stripe

UPDATE/EDIT: Talked to @vardges-stripe and we do not need to make FinancialConnections depend on StripeLinkCore. We'll do that later.

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Does the merging of this PR mean that people will have to perform migration steps IF they use StripePaymentSheet?

Since this is a little bit of a migration commitment, I wonder what you think about trying to do this once for any framework that will eventually depend on StripeLinkCore? Is it just StripePaymentSheet and then eventually FinancialConnections, or are there others? For example, I wonder if Financial Connections should do this now (even if we don't use it at the moment) because we will have to anyway. CC: @vardges-stripe

[eurias-stripe]

leave this as x.x.x yyyy-mm-dd the release script will replace it with the right version/date

[davidme-stripe]

Chatted with bmelts offline, we're holding off on a major version bump for now.