fix: issues parsing Xcode 16 projects

[pepicrft]

Resolves https://github.com/tuist/XcodeProj/issues/861

This PR contains some Xcode 16-related fixes:

• We were trying to parse PBXProject.preferredProjectObjectVersion as an integer but in Xcode 16 is encoded as a string. I'm decoding it as a string and converting it to an integer. If Apple supports strings that don't represent integers down the road we'll have to change the internal type.
• They added the attribute minimizedProjectReferenceProxies to the PBXProject object.
• SynchronizedRootGroups are encoded using multiple-lines. I adjusted the internal model to reflect that and ensure there are no diffs when we write the project back.

[pepicrft]

@tuist/community-core this is ready for a review 👀