FindBoost: Add support for Boost 1.75

Update the list of known versions.

Run the command

    cmake -DBOOST_DIR=/path/to/boost_1_75_0 \
      -P Utilities/Scripts/BoostScanDeps.cmake

to extract dependencies from the 1.75.0 source tree.
They differ from 1.74's dependencies by the addition of
dependencies of the json component, so add a new version
block to FindBoost.

Fixes: #21773