[PM-13356] Fix iOS build by pinning dotnet and MAUI version

Type of change

[x] Bug fix
[ ] New feature development
[ ] Tech debt (refactoring, code cleanup, dependency upgrades, etc)
[ ] Build/deploy pipeline (DevOps)
[ ] Other

Objective

iOS build was failing due to installing the latest MAUI version which depends on iOS 18, while using a dotnet version that doesn't support iOS 18. Pinned both dotnet sdk runtime and maui workload versions to the last that supports iOS 17.5.

Tried pinning them to the versions that we were using for the last release but (1) they weren't installing due to a bug in the dotnet version we were using and (2) it's possible that the MAUI version we had isn't available anymore.

Code changes

global.json: Pins the dotnet sdk runtime version. In build.yml, the repo checkout step had to be moved up for this file to be used build.yml:

Pins dotnet sdk runtime version and MAUI workload version to 8.0.402, the last version that supports iOS 17.5
Updates xcode version to 15.4, as required by MAUI 8.0.402

Before you submit

Please check for formatting errors (dotnet format --verify-no-changes) (required)
Please add unit tests where it makes sense to do so (encouraged but not required)
If this change requires a documentation update - notify the documentation team
If this change has particular deployment requirements - notify the DevOps team

bitwarden / mobile