Deprecate `dstSubfolderSpec` in favor of `dstSubfolder` in `PBXCopyFilesBuildPhase`

[pepicrft]

As seen here, Xcode 26 is no longer using dstSubfolderSpec, but dstSubfolder, which is a more human-readable version of the attribute.

We should deprecate the old one, and add support for the new one.