gh-128759: Fix accesses to `tp_version_tag`. (GH-129750)

We should use a relaxed atomic load in the free threading build in
`PyType_Modified()` because that's called without the type lock held.
It's not necessary to use atomics in `type_modified_unlocked()`.

We should also use `FT_ATOMIC_STORE_UINT_RELAXED()` instead of the
`UINT32` variant because `tp_version_tag` is declared as `unsigned int`.