Update Github workflow build & run destinations, fix respective integration tests

[w-goog]

All tests are updated to use macOS 15, Xcode 16.4, and iOS 18.6. This also requires an update to the integration tests, which were looking for a different title on the Password Manager field.

Ordinarily, I would see "breakage due to macos-latest/default sims/default xcode" changing as a useful reminder to update our repo. Due to an unfixed bug in the iOS 18.4 simulators and the Github runtimes update, using macOS-latest and the default simulators does not work.

More on Github runtime updates:
Github has opted to support the latest three versions of the platform tools for Xcode. If the default Xcode runtime for a macOS image is not one of those, simulators won't be available. This is currently true of macOS-15, where Xcode 16.0 is the default. As such, we need to specify the Xcode version. I've specified the macOS version as well to ensure that version of Xcode is available.

[w-goog]

All tests passed on the last run, re-running the prior failure to make sure there's not a hidden flake.

[brnnmrls]

Added a question and nit comments. Overall, looks pretty good!

[w-goog]

Interestingly, this latest failure is due to the mac image only having iOS 26 simulators?!? Fun new failure mode unlocked.