Deprecate CentOS 6

[uilianries]

Another distro reached EOL and I didn't find an old-releases equivalent for CentOS6. This situation is bad, because now we can't update our images based on CentOS, making other PRs broken.

My idea is deprecate Cent OS 6 on this PR, and open a new one replacing by Cent OS 7.

Cent OS 6 uses glibc 2.12
Cent OS 7 uses glibc 2.17

/cc @ericLemanissier

UPDATE:

• Add CentOS 7 is no longer needed, as CCI (official) doesn't use those images, only based on Ubuntu.

• Refer to the issue that supports this Pull Request.
• If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
• I've read the Contributing guide.
• I've followed the PEP8 style guides for Python code.
• I've followed the Best Practices guides for Dockerfile.

[Croydon]

I'm not sure if we actually want to add another CentOS based image. I think in the long-term it would be awesome if we could build our compilers ourselves (hello GCC and LLVM Conan packages!) and then choose an actively supported LTS distro as a base image for all containers.

The huge advantage for CentOS was the old glibc versions, making binaries compatible with pretty much every supported distribution, but we don't use it in CCI anyway and I just migrated Bincrafters away from it too

[uilianries]

I'm not sure if we actually want to add another CentOS based image. I think in the long-term it would be awesome if we could build our compilers ourselves (hello GCC and LLVM Conan packages!) and then choose an actively supported LTS distro as a base image for all containers.

The huge advantage for CentOS was the old glibc versions, making binaries compatible with pretty much every supported distribution, but we don't use it in CCI anyway and I just migrated Bincrafters away from it too

Same feeling. We don't use for CCI too, the only propose would be for community, but we don't want to maintain it we don't use directly. Indeed, adding compiler packages is the future IHMO.