Old version number in CMakeLists.txt

### Describe the bug

The old version can be seen here:  [ VERSION 2.15](https://github.com/pjsip/pjproject/blob/dbcf7fd7eba92699810e3a041c8501d939f8a172/CMakeLists.txt#L4)

### Steps to reproduce

...nothing to be reproduced....

### PJSIP version

2.16

### Context

- Toolchain

### Log, call stack, etc

```shell
... not needed....
```