chore: update to rust 1.81.0

[Stebalien]

Matches builtin actors and fixes cargo-deny.

[BigLep]

Approving per 2025-03-14 verbal with @rjan90 , @ZenGround0 , and @Stebalien

[Stebalien]

Ok, this is going to take some more work.

[Stebalien]

I'm going to need to make a few fixes here. So I'll do this in the next release and we'll just have to live with cargo-deny failing (that's fine, tbh).

[rvagg]

@Stebalien I hope you don't mind but I pushed a commit here to make it happier; I'd like to get back to tinkering with #2101 but I get errors all over my IDE due to ABI mismatches because of the old Rust version and would like to not have to deal with any of that so I'll work on top of this branch instead.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 78.57143% with 6 lines in your changes missing coverage. Please review.

Project coverage is 75.85%. Comparing base (aaf5155) to head (7728aae).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
ipld/kamt/src/bitfield.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2109      +/-   ##
==========================================
- Coverage   75.95%   75.85%   -0.11%     
==========================================
  Files         154      154              
  Lines       15772    15541     -231     
==========================================
- Hits        11980    11788     -192     
+ Misses       3792     3753      -39     

Files with missing lines	Coverage Δ
fvm/src/engine/mod.rs	86.14% <100.00%> (-0.43%)	⬇️
fvm/src/executor/mod.rs	21.21% <ø> (ø)
fvm/src/kernel/filecoin.rs	19.89% <ø> (ø)
fvm/src/syscalls/filecoin.rs	18.79% <ø> (ø)
ipld/amt/src/lib.rs	94.11% <100.00%> (ø)
ipld/hamt/src/bitfield.rs	90.97% <100.00%> (ø)
sdk/src/crypto.rs	0.00% <ø> (ø)
shared/src/address/payload.rs	75.23% <ø> (-0.24%)	⬇️
shared/src/address/protocol.rs	100.00% <ø> (ø)
ipld/kamt/src/bitfield.rs	84.71% <0.00%> (ø)

... and 22 files with indirect coverage changes

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[rvagg]

ok, clippy is really annoying to run in this environment .. 🤞

[Stebalien]

I've filed bytecodealliance/wasmtime#10454. Our use of features here wasn't effective anyways.