cmake: Downgrade deprecation error for GCC compilations

[PatTheMav]

Description

Downgrades the deprecated declarations error to warnings for GCC compilations in line with Clang and AppleClang.

Motivation and Context

Error has already been downgraded for Clang and AppleClang, not doing so for GCC was an oversight.

How Has This Been Tested?

Tested OS(s): Ubuntu 24.04

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the Contributing Guidelines.
• All commit messages are properly formatted and commits squashed where appropriate.
• My code is not on master or a release/* branch.
• The code has been tested.
• I have included updates to all appropriate documentation.