breaking: drop C++ 11 (deepmodeling#4068)

Fix deepmodeling#4060.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Documentation**
- Updated installation documentation to clarify C++ compiler
requirements, specifying that GCC 5 or higher is necessary for C++ 14
and potentially GCC 7 for C++ 17.
  
- **Chores**
- Enhanced CMake configuration to support C++ 14 in specific scenarios,
improving compatibility with older codebases.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

doc/install/install-from-source.md

@@ -108,7 +108,8 @@ Check the compiler version on your machine
 gcc --version
 ```
 
-The compiler GCC 4.8 or later is supported in the DeePMD-kit.
+By default, DeePMD-kit uses C++ 14, so the compiler needs to support C++ 14 (GCC 5 or later).
+The backend package may use a higher C++ standard version, and thus require a higher compiler version (for example, GCC 7 for C++ 17).
 
 ::::{tab-set}
 

source/CMakeLists.txt

@@ -16,6 +16,10 @@ macro(set_if_higher VARIABLE VALUE)
     set(${VARIABLE} ${VALUE})
   endif()
 endmacro()
+if(NOT DEEPMD_C_ROOT)
+  # we can still allow C++ 11 for programs linked to the C library
+  set_if_higher(CMAKE_CXX_STANDARD 14)
+endif()
 
 if(BUILD_TESTING)
   enable_testing()