Fix sendability warnings in new Swift 6 toolchains

[hallee]

In Xcode 16 Beta 3 (and also the latest Swift 6 toolchain), there are new data race errors that cause this library to not compile with strict checking enabled:

 54 |         do {
 55 |           for try await element in sequence {
 56 |             continuation.yield(element)
    |                          |- error: sending 'element' risks causing data races
    |                          |- note: 'element' used after being passed as a 'sending' parameter; Later uses could race
    |                          `- note: access can happen concurrently
 57 |           }
 58 |           continuation.finish()

/Users/hal/Developer/swift-dependencies/Sources/Dependencies/Internal/Deprecations.swift:85:26: error: sending 'element' risks causing data races
 83 |         do {
 84 |           for try await element in sequence {
 85 |             continuation.yield(element)
    |                          |- error: sending 'element' risks causing data races
    |                          |- note: 'element' used after being passed as a 'sending' parameter; Later uses could race
    |                          `- note: access can happen concurrently
 86 |           }
 87 |           continuation.finish()

Adding a Sendable constraint for Element seems to silence the errors.

[mbrandonw]

Thanks @hallee! We'll get a release out later today.