Update `heapless` to resolve `RUSTSEC-2023-0089`

[BatmanAoD]

chacha20poly1305 depends on heapless = "^0.7", but that version depends on atomic-polyfill, which is unmaintained and deprecated.

v0.8.0 of heapless depends on the replacement crate, portable-atomic.

[Shadow53]

This appears to be updated already in aead and the actual fix here, I think, is to create a new release of aead and update it here.

[newpavlov]

Updating heapless in aead v0.5 would be a breaking change, so we can not do it. aead v0.6 is currently under development and it will be released with the updated heapless dependency together with crates from this repository in the relatively near future (we do not have a concrete date yet).

If you want to get a fix early, you either can use the pre-release versions, or patch the aead dependency for your project.