Cleanup enabling IPO in CMake

[ranisalt]

Pull Request Prelude

• [x] I have followed proper The Forgotten Server code styling.
• [x] I have read and understood the contribution guidelines before making this PR.
• [x] I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

Cleanup enabling IPO (LTO) by setting the global value CMAKE_INTERPROCEDURAL_OPTIMIZATION which sets the default value for INTERPROCEDURAL_OPTIMIZATION in all targets rather than enabling in each target.

Additionally, CMP0069 does not need to be set to NEW since we require a minimum version of CMake 3.19, while it is already set to NEW when the minimum required version is 3.9 or higher.