cmake: Set minimum version for nlohmann_json to 3.11

[RytoEX]

Description

We know this is the minimum version that we require, so we can just fail during configure if somehow an older version is found.

Motivation and Context

See:

• https://github.com/obsproject/obs-browser/pull/452
• https://github.com/obsproject/obs-studio/pull/11384

Although we no longer support Ubuntu 22.04 for building or running OBS Studio, we can make the dependency requirement here more obvious at configure time.

How Has This Been Tested?

Tested OS(s):

• Windows 11

Types of changes

• Code cleanup (non-breaking change which makes code smaller or more readable)

• Other Enhancement (anything not applicable to what is listed)

Checklist:

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