Bump thirdparty/zlib from ac8f12c to 15c45ad

Bumps [thirdparty/zlib](https://github.com/madler/zlib) from `ac8f12c` to `15c45ad`.
- [Release notes](https://github.com/madler/zlib/releases)
- [Commits](madler/zlib@ac8f12c...15c45ad)

---
updated-dependencies:
- dependency-name: thirdparty/zlib
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>