chore(gpu): use byte add, update rust msrv

[agnesLeroy]

closes: https://github.com/zama-ai/tfhe-rs-internal/issues/549

PR content/description

Check-list:

• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)
• [ ] Relevant issues are marked as resolved/closed, related issues are linked in the description
• [ ] Check for breaking changes (including serialization changes) and add them to commit message following the conventional commit specification

[tmontaigu]

We could use https://doc.rust-lang.org/std/primitive.pointer.html#method.wrapping_byte_add-1 which does the same thing but is not unsafe

[agnesLeroy]

@tmontaigu I just pushed the change.