Update Bitwuzla to 0.8.1

[baierd]

Bitwuzla recently released version 0.8.1.

This includes bug-fixes, general improvements, and some SMTLib2 parser improvements, as well as converting a floating-point value to a Real string.

We have to be careful as a new Abstraction module for BVs is included and seems to be enabled per default!

Note: this should be done in conjunction with #417.

[kfriedberger]

The update of Bitwuzla is done in #500