Trivial: Add version requirement for nlohmann/json

[stephematician]

Description

Adds a version requirement for nlohmann/json to find_package call in CMakeLists.txt.

Motivation and Context

obs-browser requires nlohmann/json >= 3.11 due to the use of *_WITH_DEFAULT macros, which are not defined in earlier versions.

How Has This Been Tested?

Tested on UbuntuMATE 22.04, g++12.

• nlohmann/json built and installed from source;
• used backport of ffmpeg 6 from ppa.

Types of changes

Build improvement: Makes it easier to identify problems when nlohmann < 3.11 installed - likely not critical as e.g. linux distros with ffmpeg6 in APT almost always have nlohmann >= 3.11; but 22.04 (for example) does not.

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.