Drop C++14 Support

[ken-matsui]

IMO, there is no need to provide further support for C++14 like cpr has already mandated the use of C++17 for its building requirements.

https://github.com/libcpr/cpr/releases/tag/1.9.0

Expected Outcome

• Some header files can be eliminated since they are the same as the standard library headers.
  • lib/inc/drogon/utils/optional.h
  • lib/src/filesystem.h
  • lib/inc/drogon/utils/string_view.h
  • lib/inc/drogon/utils/any.h
• I can see that we should be able to remove "boost" from our dependencies entirely.
  • https://github.com/search?q=repo%3Adrogonframework%2Fdrogon%20boost&type=code
• It's possible to simplify certain codes that involve macros for C++17.
  • https://github.com/drogonframework/drogon/blob/8b250f8638d526f4d766c1ae710808acb93044f7/lib/inc/drogon/utils/apply.h#L16
  • https://github.com/drogonframework/drogon/blob/8b250f8638d526f4d766c1ae710808acb93044f7/lib/inc/drogon/PubSubService.h#L39

Expected Drawbacks

• Drogon will not be able to build with C++14 compatible compilers.

Tasks

• [ ] Update CMakeLists.txt to use only C++17 or later: https://github.com/drogonframework/drogon/pull/1712
• [ ] Delete lib/inc/drogon/utils/optional.h and use the standard library instead
• [ ] Delete lib/src/filesystem.h and use the standard library instead
• [ ] Delete lib/inc/drogon/utils/string_view.h and use the standard library instead
• [ ] Delete lib/inc/drogon/utils/any.h and use the standard library instead
• [ ] Remove Boost from our depenedencies
• [ ] Simplify some codes with macros for C++17.
  • https://github.com/drogonframework/drogon/blob/8b250f8638d526f4d766c1ae710808acb93044f7/lib/inc/drogon/utils/apply.h#L16
  • https://github.com/drogonframework/drogon/blob/8b250f8638d526f4d766c1ae710808acb93044f7/lib/inc/drogon/PubSubService.h#L39

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Related: https://github.com/drogonframework/drogon/pull/1331#issuecomment-1199819795

[formbook]

please

[ken-matsui]

https://github.com/drogonframework/drogon/pull/1740