Remove old compiler workaround in src/compatibility.h

[BrettDong]

Summary

Build "Remove old compiler workaround in src/compatibility.h"

Purpose of change

Closes #36370. src/compatibility.h is a workaround to old GCC prior to 4.8 lacking std::to_string and std::advance. But now the minimum supported version is GCC 5.3 (see https://github.com/CleverRaven/Cataclysm-DDA/blob/master/doc/COMPILING/COMPILER_SUPPORT.md), so this workaround is no longer needed.

Describe the solution

• First (930a939) migrate all references to proxy function to_string() defined in src/compatibility.h to real std::to_string
• Then (84f51d7) remove src/compatibility.h

Testing

Compiles locally for me. Wait and see if this compiles on all CI environments as well.

So far GitHub Action and Travis CI checks have all passed but AppVeyor failed because of time out and there isn't a rebuild button on AppVeyor.

[BrettDong]

Doing a force-push to retrigger timed-out AppVeyor build.

[BrettDong]

AppVeyor still times out.

[actual-nh]

@ZhilkinSerg, @BrettDong: Should I put up an issue for the Appveyor builds timing out?

[ZhilkinSerg]

We need a branch that we can use to refresh vcpkg package cache, we can ask Appveyor to increase timeout to 90 minutes (I've seen they do that on request), we can disable building and/or tests.

Maybe think of a way of storing vcpkg cache somewhere or use a specific vcpkg commit, so we won't need to rebuild packages most of the time.