Require sendable userInfo values in JSON.withEncoding(of:userInfo:_:)

[stmontgomery]

swift-foundation recently landed a change (in swiftlang/swift-foundation#764) which requires any Sendable values in JSONEncoder.userInfo. This causes a build failure in swift-testing:

JSON.swift:44:28: error: type 'Any' does not conform to the 'Sendable' protocol 
42 | 
43 |     // Set user info keys that clients want to use during encoding.
44 |     encoder.userInfo.merge(userInfo, uniquingKeysWith: { _, rhs in rhs})
   |                            `- error: type 'Any' does not conform to the 'Sendable' protocol

This PR adjusts our userInfo: parameter require any Sendable values. The values we were passing to this utility were already sendable, luckily.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[stmontgomery]

@swift-ci please test

[tshortli]

Reverted in #957 due to Swift CI breakage.