Reduce required version of gcc to 4.8 per latest project README

[headupinclouds]

• I've checked this Git style guide. [Yes]
• I've checked this CMake style guide. [Yes]
• My change will work with CMake 3.2 (minimum requirement for Hunter). [Yes]
• I will try to keep this pull request as small as possible and will try not to mix unrelated features. [Yes]

Relax gcc compiler requirements -- the package currently works with GCC 4.8 or newer.

https://github.com/nlohmann/json#supported-compilers

• GCC 4.8 - 9.0 (and possibly later)

Updating this for the latest version seems to be the right thing to do. It might be the case that older versions had different requirements.