Upgrade to Conan 2

[kyllingstad]

This fixes issue #718, “Conan v2 compatibility”. (Note that it also destroys Conan 1.x compatibility – it was simply too difficult to maintain support for both major versions.)

Some collateral changes warrant further explanation:

• The modified <gsl/util> includes and various changes from int to std::size_t are a consequence of using a newer version of MS GSL. I changed the dependency version so we can use an upstream CMake package configuration file which is compatible with the one generated by Conan 2's CMakeDeps generator. This should enable us to use the same find_package() commands and target names regardless of whether the build is running under Conan or not.
• I've modified the “non-Conan” CI workflow to build in a clean Docker container like the Conan CI workflow. I've based the container on Debian Bookworm, whose packages for our dependencies are new enough that all provide CMake package configuration files which match those generated by CMakeDeps, which was my main motivation. The MS GSL package mentioned in the previous point is one such. (Also, the GitHub runners are such a noisy and unpredictable build environment that I figured this would be more efficient than try to fix compatibility issues there.)
• I've had to remove the option to run Conan from within CMake, because upstream hasn't released a version of the cmake-conan helper script with full support for all Conan 2 features yet. (Notably, support for the CMakeToolchain generator is missing.)

The pull request depends on open-simulation-platform/proxy-fmu#92, which should be merged first. Then, the osp/testing-… channel should be replaced in the proxyfmu requirement specification in conanfile.py before merging the present PR.

Incidentally, this PR also fixes issue #702.

[kyllingstad]

Seems like this PR closes several build-related issues. I linked them now.

[davidhjp01]

@kyllingstad @restenb

• Wondering if we should remove Debug builds because it seems the case for the most of the libraries in conan2 public repository (to avoid lengthy github action builds).
• Also for some reason conan2 tries to rebuild some of the libraries for the Release builds, i guess due to some mismatching conan2 options?

Edit It is because there are no gcc-9 builds available in conancenter

[davidhjp01]

I have addressed the most of the issues related to the failing builds. I suggest to merge and release libcosim and proxyfmu so that other projects dependent on conan2 library can consume them.

[kyllingstad]

@davidhjp01, sorry for dropping the ball on this at the end of last year, and huge thanks to you for picking it up! It's really good to finally get this merged.

[kyllingstad]

Following up on your question (too late for this PR, unfortunately):

Wondering if we should remove Debug builds because it seems the case for the most of the libraries in conan2 public repository (to avoid lengthy github action builds).

Yes, I think that sounds like a good idea. I don't think I can remember a case where the release builds succeeded while the debug builds failed or vice versa.