switch to c++17

[simogasp]

Switch to c++17 standard as recent versions of GTest require it. The CI on macos was failing because homebrew pulls such versions.

Incidentally,

• added permissions to the workflow
• set a monthly schedule for automatically triggering the workflow

[simonfuhrmann]

Can you explain this section to me? Why does this workflow need non-default permissions? What does checks: write and pull-requests: write mean?

[simogasp]

Apparently, it's a best practice to make the permissions explicit (https://codeql.github.com/codeql-query-help/actions/actions-missing-workflow-permissions/)
What I did wrong here is a bad copy-paste from another repo of mine in which I had codecov and dependabot that required the write permissions.

Sorry about that, I will fix it to be read-only, unless you are planning to use any tools like that.

Also, if you agree, I'd remove the old pipelines like ubuntu 20.04 and macos 13 as they either fail while setting up (macos) or get stuck. You can see it on my fork here
https://github.com/simogasp/mve/actions/runs/19043320892

[simonfuhrmann]

Thanks.