Update openexr to 3.1.8

[0mdc]

Motivation and Context

The current version of openexr fails to build on gcc 13. This was fixed in version 3.1.6.

This commit bumps openexr to the latest version to benefit from this fix as well as other improvements.

How Has This Been Tested

Tested locally and on CI.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.