Restore the ability to pass in customXCTestMinimumDeploymentTargets when loading a package graph (#4254)

This had been removed in one of the previous refactorings, but clients of libSwiftPM still need it.

Author: abertelrud

Sources/Workspace/Workspace.swift

@@ -1189,6 +1189,7 @@ extension Workspace {
         createMultipleTestProducts: Bool = false,
         createREPLProduct: Bool = false,
         forceResolvedVersions: Bool = false,
+        customXCTestMinimumDeploymentTargets: [PackageModel.Platform: PlatformVersion]? = .none,
         observabilityScope: ObservabilityScope
     ) throws -> PackageGraph {
 
@@ -1225,6 +1226,7 @@ extension Workspace {
             binaryArtifacts: binaryArtifacts,
             shouldCreateMultipleTestProducts: createMultipleTestProducts,
             createREPLProduct: createREPLProduct,
+            customXCTestMinimumDeploymentTargets: customXCTestMinimumDeploymentTargets,
             fileSystem: fileSystem,
             observabilityScope: observabilityScope
         )