Fix version search for zlib since 1.3 is now released

Issue/feature addressed

The version search for the zlib library means that the latest zlib release (1.3.1) is not detected by CMake.

Proposed solution

Change the version search to use the full ZLIB_VERSION_STRING

Implementation

Tests

Suggested reviewers

@ccantwel

Notes

Please add any other information that could be useful for reviewers.

Checklist

• Functions and classes, or changes to them, are documented.
• [ ] User guide/documentation is updated.
• Changelog is updated.
• [ ] Suitable tests added for new functionality.
• [ ] Contributed code is correctly formatted. (See the [contributing guidelines](https://gitlab.nektar.info/nektar/nektar/-/blob/master/CONTRIBUTING.md#using-clang-format)).
• [ ] License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).