Replace -require-explicit-sendable with 'ExplicitSendable' warning diagnostic group

[stmontgomery]

This replaces usage of the -require-explicit-sendable compiler flag with the new ExplicitSendable diagnostic group which was recently introduced in swiftlang/swift#85319, with warning severity.

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]

Marking as Draft initially because I intend to wait to merge this until some of our downstream systems have upgraded to a newer toolchain which recognizes this diagnostic group.