Increase MSRV to 1.80 when 1.90 is released

[marvin-j97]

Tracking issues for all changes that require a higher MSRV than 1.74:

• 1.76 triomphe Arc (also value_log)
• 1.76 inspect_err: https://github.com/search?q=repo%3Afjall-rs%2Flsm-tree%20inspect_err&type=code
• 1.77 Bound::map: https://github.com/search?q=repo%3Afjall-rs%2Flsm-tree+Bound%3A%3Amap&type=code
• 1.79 std::path::absolute: https://github.com/search?q=repo%3Afjall-rs%2Flsm-tree+std%3A%3Apath%3A%3Aabsolute&type=code - removes path_absolutize dependency, same in value_log and fjall
• 1.80 IntoIter for Box<[T]> (full block index)