clippy: disable warning for repeat_n

with recent rustc version we can use `repeat_n` instead of `repeat().take()`

```
error: this `repeat().take()` can be written more concisely
   --> lightning-invoice/src/ser.rs:256:12
    |
256 |         Box::new(core::iter::repeat(Fe32::Q).take(to_pad).chain(fes))
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using `repeat_n()` instead: `core::iter::repeat_n(Fe32::Q, to_pad)`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_repeat_n
    = note: `-D clippy::manual-repeat-n` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::manual_repeat_n)]`
```

but to keep compatibility with older rustc versions we need to disable the warning
for the `repeat_n` lint.

Signed-off-by: Vincenzo Palazzo <[email protected]>

Author: vincenzopalazzo

ci/check-lint.sh

@@ -94,4 +94,5 @@ RUSTFLAGS='-D warnings' cargo clippy -- \
 	-A clippy::unnecessary_unwrap \
 	-A clippy::unused_unit \
 	-A clippy::useless_conversion \
-	-A clippy::unnecessary_map_or `# to be removed once we hit MSRV 1.70`
+	-A clippy::unnecessary_map_or `# to be removed once we hit MSRV 1.70` \
+	-A clippy::iter_repeat_n `# to be removed once we hit MSRV 1.86`