Raise CMake requirements to 3.16

This is what the Dune core modules currently require.

I expect CI to fail, because the images we are testing with are too old. Will update the CI script once there are newer clang images.

added 2 commits

• 918d02e6 - Stop testing with Debian-10 images
• 0cc504c6 - Add a 'codespell' CI job to check for spelling errors

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added 1 commit

• 398b0d56 - Add a 'codespell' CI job to check for spelling errors

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mentioned in commit d41f514a

merged