ENH: Add lazy copy for truncate

[phofl]

• xref ENH / CoW: Use the "lazy copy" (with Copy-on-Write) optimization in more methods where appropriate #49473 (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

[phofl]

This is now covered in the cow tests

[jorisvandenbossche]

This might not be needed, since the result is from an indexing operation, and already should be a lazy copy (when using a slice) with refs set up

[phofl]

We could rewrite as if copy or copy is None and _using_copy_on_write(), but we need the check for the non cow case

[jorisvandenbossche]

Should it be if copy or (copy is None and not _using_copy_on_write())? (so with brackets and when not using CoW)

Essentially we need to same behaviour as this pattern that is used elsewhere, I think:

        if copy is None:
            if _using_copy_on_write():
                copy = False
            else:
                copy = True
        if copy:
            result = result.copy()

[phofl]

Ah yes, forgot the not