Update minimum partition sizes for bookworm
The sizes are based on empirical data obtained by installing all task combinations which are also documented in the README file. Also, fix some smaller issues in the Makefile.
This addresses bug #1038792.
After merging, https://wiki.debian.org/DebianEdu/Documentation/Bookworm/Requirements should be updated based on the output of make minimum-diskreq
as the figures given there are no longer accurate.