iwa: Use IwaVersion in UpdateDiscoveryTask

This CL refactors `UpdateDiscoveryTask` and affected files to replace
`base::Version` with the more specific `IwaVersion` type. Notably, the
`currently_installed_version_` in `IsolatedWebAppUpdateDiscoveryTask`
was changed from `base::Version` to `std::optional<IwaVersion>` as
`IwaVersion` cannot be default-constructed to an invalid state.

Bug: 437038363
Change-Id: I0033de44ec24af61e9c4e8190db04bb41ce0ec85
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6859121
Reviewed-by: Greg Thompson <grt@chromium.org>
Reviewed-by: Patryk Chodur <pchodur@google.com>
Commit-Queue: Paulina Gacek <paulinagacek@google.com>
Cr-Commit-Position: refs/heads/main@{#1505829}