Require specific t8code version

[holke]

When DLR-AMR/t8code#2099 is merged and released, we should require this version of t8code as minimum version,

Probably change the find_package ( T8CODE REQUIRED) in CMakeLists.txttofind_package ( T8CODE 26.01 REQUIRED)` or similar, depending on the final version number of the next release containing DLR-AMR/t8code#2099