Prepare Rust crates for 0.7.5 release

[Matthew Parkinson (mjp41)]

Summary

• bump snmalloc-rs and snmalloc-sys to 0.7.5
• update the wrapper dependency requirement to 0.7.5
• update the Rust changelog and README release metadata

Validation

• Debug build and 80/80 C++ tests passed on the 0.7.5 baseline
• cargo build --all passed
• 12 Rust tests passed
• package content checks passed for both crates
• cargo publish --workspace --exclude xtask --dry-run passed from the committed tree