build: Bump required mono version to 2.8.2

The required version had not been updated in quite a while, and was
probably a lie...

Bump to the mono version required by Banshee, which is the version
checked when building Hyena as part of Banshee.
Stage: Remove workaround for a bug in old mono compiler

The bug is fixed in mono version 2.8.2 that we now require.
build: Re-enable disabled warnings during compilation

Those warnings were disabled a long time ago to work around bugs in old
mono compilers.