chore: bump sysinfo to 0.39 and raise MSRV to Rust 1.95 (#258)

Bump sysinfo 0.38 -> 0.39 (which requires MSRV 1.95) and move the Docker build base and all Launchpad PPA toolchain references to Rust 1.95. No source changes were needed: sysinfo 0.39's only breaking change is the MSRV bump.

Closes #257