Bump rust to 1.78.0 in actions runner

[jpculp]

Description of changes:

Bumps rust to 1.78.0 and addresses the clippy warnings that came with it.

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[bcressey]

We need to update cargo deny to fix this error:

cargo deny --all-features check --disable-fetch licenses bans sources
thread '<unnamed>' panicked at /home/runner/work/tough/tough/.cargo/registry/src/index.crates.io-6f17d22bba15001f/krates-0.14.0/src/builder.rs:791:47:
called `Option::unwrap()` on a `None` value

I'd prefer to keep _extra as-is and just add a lints table to the workspace Cargo.toml to allow it.

[jpculp]

• Rebased on Update rust dependencies (including AWS SDK) #781 (which updates cargo-deny).
• Added allow(clippy::pub_underscore_fields) to tough/src/schema/mod.rs.

[bcressey]

I am surprised that so many errors end up unused.

Over in the core kit, when I was working on signpost, I noticed that a lot of the new "unused" warnings were false positives. Seems to be an issue with a crate that's built as both a library and a binary - the error variants that aren't used in the library side get flagged even though the binary needs them.

Which is to say, please double check via git grep or rg that all of these are actually not used.

[jpculp]

I think a couple of these might have just been copied from tough at some point. I wasn't able to find any references in tuftool itself.