chore: update pinned nightly to `nightly-2025-07-21`

This commit updates the pinned nightly toolchain to `nightly-2025-07-21`. This was fairly straightforward, but required the following changes: - Changing the `x86_64-mycelium.json` target spec to explicitly request the `x86-softfloat` ABI, which is required as we are disabling SSE and MMX and enabling soft-floats (593b3d02d68ce1737100851ce9a11615bc33ee3b). - Updating the `bootloader` crate to v0.11.10, which also has a target that specifies the `x86-softfloat` (ABI 9cf55ffa7ba8cf8af68dc1aa8a79c4c715e8e7fb4) - Fixing a bunch of new warnings and Clippy lints (`must_use` does nothing on trait methods, docs list indentation nonsense) I've built and run an x86 kernel and it still works fine.