The recent changes related to adding support for working beatmap load
cancellation exposed a flaw in the beatmap difficulty cache. With the
way the difficulty computation logic was written, any error in the
calculation process (including beatmap load timeout, or cancellation)
would result in a 0.00 star rating being permanently cached in memory
for the given beatmap.
To resolve, change the difficulty cache's return type to nullable.
In failure scenarios, `null` is returned, rather than
`default(StarDifficulty)` as done previously.
I am aware there are more throughout the codebase but intentionally left
the remaining mentioned for one reason or another. The intention here is
to mainly change user-facing versioning to change the positioning of the
"lazer" term (to be where we would expect "cuttingedge" or "beta" to
be).