rust: give Clippy the minimum supported Rust version

Clippy's lints may avoid emitting a suggestion to use a language or
library feature that is not supported by the minimum supported version,
if given by the `msrv` field in the configuration file.

For instance, Clippy should not suggest using `let ... else` in a lint
if the MSRV did not implement that syntax.

If the MSRV is not provided, Clippy will assume all features are available.

Thus enable it with the minimum Rust version the kernel supports.

Note that there is currently a small disadvantage in doing so: since
we still use unstable features that nevertheless work in the range
of versions we support (e.g. `#[expect(...)]`), we lose suggestions
for those. However, over time we will stop using unstable features
(especially language and library ones) as it is our goal, thus, in the
end, we will want to have the `msrv` set.

Rust is also considering adding a similar feature in `rustc` too, which
we should probably enable if it becomes available [2].

Link: https://github.com/rust-lang/rust-clippy/blob/8298da72e7b81fa30c515631b40fc4c0845948cb/clippy_utils/src/msrvs.rs#L20 [1]
Link: rust-lang/compiler-team#772 [2]
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20241123222345.346976-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

.clippy.toml

@@ -1,5 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0
 
+msrv = "1.78.0"
+
 check-private-items = true
 
 disallowed-macros = [