Pin older iOS versions when building Sentry.Maui

[jamescrosswell]

See this discord conversation.

It appears if we simply specify net8.0-ios and net9.0-ios, we'll get different build results depending on what the latests workload installed on the machine running the build is.

Rolf's recommendation (which might enable us to create more deterministic builds, so we get the same results on CI and dev boxes):

set TargetFramework=net8.0-ios17.0 in your library project, that'll work for all .NET 8 iOS workloads and TargetFramework=net9.0-ios18.0 for .NET 9 (although net8.0-ios17.0 works in .NET 9 as well)

[bricefriha]

Sounds good, I'll try that