Prepare Rust crates for 0.7.6 release

[Matthew Parkinson (mjp41)]

Summary

• bump snmalloc-rs and snmalloc-sys to 0.7.6
• update the wrapper dependency requirement to 0.7.6
• update the Rust changelog and README release metadata

Publishing the 0.7.6 crates should wait until the snmalloc 0.7.6 tag and GitHub release are created from the release baseline.

Validation

• Debug build and 88/88 C++ tests passed on the 0.7.6 baseline
• cargo build --all passed
• 12 Rust tests passed
• package content checks passed for both crates (205 snmalloc-sys files and 10 snmalloc-rs files, unchanged from 0.7.5)
• cargo publish --workspace --exclude xtask --dry-run packaged and verified both 0.7.6 crates from the committed tree
• fresh-context implementation review approved the final commit