Switch default C++ standard to C++17

[dbaston]

What does this PR do?

Updates CMakeLists.txt to use C++17 by default.

What are related issues/pull requests?

#8270

Tasklist

• Review
• Adjust for comments
• All CI builds and checks have passed

[dbaston]

The Windows build is currently failing to build the libkml driver, due to an old version of boost packaged in http://download.gisinternals.com/query.html?content=filelist&file=release-1911-x64-dev.zip trying to use std::auto_ptr:

https://github.com/dbaston/gdal/actions/runs/6811091438/job/18520712504#step:8:2788

[rouault]

CC @szekerest We are trying to update GDAL C++ version to C++17 and are running in a build issue with the gisinternals.com SDK. The issue is when building the LIBKML driver. libkml depends on boost, and the boost version shipped in gisinternals.com appears to use outdated C++ features like std::auto_ptr which have been removed. Could you try to update the boost version in your SDKs ?

[coveralls]

coverage: 67.869% (-0.004%) from 67.873%
when pulling 1700d0a on dbaston:cpp17
into 170bc25 on OSGeo:master.

[rouault]

Could you try to update the boost version in your SDKs ?

filed as gisinternals/buildsystem#221