Port to GridTools 2.0

[fthaler]

Port to GridTools 2.0. Further changes:

• Removed architecture-dependent block-size tuning, may be reintroduced as presets or tunable CMake-variables in the future.
• Updated CI to CUDA 11.0 and Ubuntu 20.04 LTS.
• Introduced HIP-builds in the CI setup.
• Renamed resulting Docker image tags to be consistent with those changes.
• Use Docker BuildKit for better caching support and thus faster CI.