CMake compatibility

**Build Error Description**

(This is also listed in #2348)

Note: it's a warning but not an error.

>2021-04-22T11:17:19.9061879Z CMake Deprecation Warning at libraries/fc/vendor/websocketpp/CMakeLists.txt:9 (cmake_minimum_required):
2021-04-22T11:17:19.9063442Z Compatibility with CMake < 2.8.12 will be removed from a future version of
2021-04-22T11:17:19.9064271Z CMake.
2021-04-22T11:17:19.9064717Z
2021-04-22T11:17:19.9065485Z Update the VERSION argument value or use a ... suffix to tell
2021-04-22T11:17:19.9066516Z CMake that the project does not need compatibility with older versions.

**Build Environment**
Details about the build environment, including the relevant required libraries. Much of this information can be found in the `CMakeFiles/CMakeOutput.log`.
- Host OS: [e.g. Ubuntu 18.04 LTS]
- Host Physical RAM [e.g. 4GB]
- Source Branch/Tag: [e.g. master or 2.0.180425]
- OpenSSL Version: [e.g. 1.1.0g]
- Boost Version: [e.g. 1.65.1]
- C++ Compiler: [e.g. gcc version 4.8.5]

## CORE TEAM TASK LIST
- [ ] Evaluate `Build Error`
- [ ] Provide build guidance
- [ ] Create `Bug Report`

Start at libraries/fc/vendor/websocketpp/CMakeLists.txt:9 and inspect the cmake_minimum_required declaration that emits the warning. Re-run the affected CMake configuration or build and confirm the deprecation warning is resolved; check #2348 for overlapping context.

Written by the indexing model from the issue text.