[cmake] FindRapidJSON update to newer module standards

[fuzzard]

Description

Update FindRapidJSON to newer "standard" usage ive been implementing.

Motivation and context

This allows us to find an existing RapidJSON and not "rebuild" every time for platforms. Its a header only lib, so build time isnt really a factor, but it just feels better to standardise on what ive settled on for finding/assessing existing libs and internal builds.

We also remove the hardcoded spec requirement back into the root CMakeLists.txt file. Im endeavouring to have any hard version requirements listed in the one place, rather than scattered throughout a bunch of find modules. Should make it easier for outsiders to quickly see minimum dependency versions

How has this been tested?

Macos aarch64 - with and without existing rapidjson cmake config/headers
windows x64 - with and without existing rapidjson cmake config/headers

What is the effect on users?

N/A

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• Student submission (PR was done for educational purposes and will be treated as such)
• None of the above (please explain below)

Checklist:

• My code follows the Code Guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the Contributing document
• I have added tests to cover my change
• All new and existing tests passed

[sundermann]

Works nicely for me and saves some time on each recompile