Move CURRENT_NIGHTLY_VERSION to 2.14.0 (#8226)

## Summary

2.13 has branched (`release/2.13`), so `main`'s nightly should now be
**2.14.0**. Bumps `CURRENT_NIGHTLY_VERSION` in
`tools/scripts/generate_binary_build_matrix.py`:

```python
-CURRENT_NIGHTLY_VERSION = "2.13.0"
+CURRENT_NIGHTLY_VERSION = "2.14.0"
```

## Effect

`CURRENT_NIGHTLY_VERSION` is consumed by
`tools/scripts/generate_docker_release_matrix.py` to tag the nightly
docker image (`pytorch/pytorch-nightly:<version>.dev<date>`). It is
**not** part of the generated binary build matrix (the matrix's
`stable_version` comes from `CURRENT_STABLE_VERSION`), and there is no
docker-release-matrix unit test.

`CURRENT_CANDIDATE_VERSION` and `CURRENT_STABLE_VERSION` are
intentionally left unchanged (this PR only moves nightly, per request).

## Tests

- `python -m unittest tools.tests.test_generate_binary_build_matrix` ->
7/7 pass, unchanged.
- No test assets change: regenerating `tools/tests/assets/*.json`
produces no diff, since those are nightly-channel matrices whose
`stable_version` is `CURRENT_STABLE_VERSION` (2.12.1), independent of
`CURRENT_NIGHTLY_VERSION`.

Author: atalman

tools/scripts/generate_binary_build_matrix.py

@@ -84,7 +84,7 @@
 XPU = "xpu"
 
 
-CURRENT_NIGHTLY_VERSION = "2.13.0"
+CURRENT_NIGHTLY_VERSION = "2.14.0"
 CURRENT_CANDIDATE_VERSION = "2.12.1"
 CURRENT_STABLE_VERSION = "2.12.1"
 CURRENT_VERSION = CURRENT_STABLE_VERSION