[PackageLoading] Support deserialization of manifests that used swiftLanguageVersion

Follow-up to #7620

This is important for the manifest cache - make sure that older manifests
that used original spelling of the build setting are still deserializable.

(cherry picked from commit 5189086)

Author: xedin

Sources/PackageLoading/ManifestJSONParser.swift

@@ -535,7 +535,7 @@ extension TargetBuildSettingDescription.Kind {
         case "unsafeFlags":
             return .unsafeFlags(values)
 
-        case "swiftLanguageMode":
+        case "swiftLanguageVersion", "swiftLanguageMode":
             guard let rawVersion = values.first else {
                 throw InternalError("invalid (empty) build settings value")
             }