Update version numbers for TensorFlow 2.15.0
Before merging this PR, please double check that it has correctly updated
`core/public/version.h`, `tools/pip_package/setup.py`, and
`tensorflow/tensorflow.bzl`. Also review the execution notes below:

```
Major: 2 -> 2
Minor: 15 -> 15
Patch: 0 -> 0

No lingering old version strings "2.15.0-rc1" found in source directory 
"tensorflow/". Good.
No lingering old version strings "2.15.0rc1" found in source directory 
"tensorflow/". Good.
```