What this MR does / why does DuMux need it:
Simplifies installation, CI and CMake code by assuming a dune version of 2.9 or newer. Fixes #1225 (closed)
Checking for the existence of the virtual environment is still necessary as dune 2.10 introduces the ability to deactivate it, which is used in our minimal installation pipelines.
Before you request a review from someone, make sure to revise the following points:
CHANGELOG.md
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character. Make sure there is no \ No newline at end of file
comment in "Changes" of this MR.