rust: give Clippy the minimum supported Rust version

Clippy has lints that refer to features in the language and library that
may not exist in the minimum version a given project supports. Thus it
offers the `msrv` field to avoid that.

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

Currently, there should be no effect setting it, since there is no lint
that we currently enable that is affected by `msrv` [1] lower than our
minimum supported Rust version (1.78.0).

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

Nevertheless, enable it.

Link: https://github.com/rust-lang/rust-clippy/blob/8dd459d4c750dbf200bbd48a10f129b1401e7bf3/clippy_config/src/msrvs.rs#L19 [1]
Link: rust-lang/compiler-team#772 [2]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

.clippy.toml

@@ -1 +1,3 @@
 # SPDX-License-Identifier: GPL-2.0
+
+msrv = "1.78.0"