fix(deps): update rust crate counter to 0.6.0 (#37)

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This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [counter](https://togithub.com/coriolinus/counter-rs) | dependencies |
minor | `0.5.7` -> `0.6.0` |

---

### Release Notes

<details>
<summary>coriolinus/counter-rs (counter)</summary>

###
[`v0.6.0`](https://togithub.com/coriolinus/counter-rs/releases/tag/v0.6.0)

[Compare
Source](https://togithub.com/coriolinus/counter-rs/compare/v0.5.7...v0.6.0)

#### What's Changed

- update edition, add `counter` keyword by
[@&#8203;chris-ha458](https://togithub.com/chris-ha458) in
[https://github.com/coriolinus/counter-rs/pull/34](https://togithub.com/coriolinus/counter-rs/pull/34)
- refactor tests and impls into distinct modules by
[@&#8203;chris-ha458](https://togithub.com/chris-ha458) in
[https://github.com/coriolinus/counter-rs/pull/36](https://togithub.com/coriolinus/counter-rs/pull/36)
- small doc formatting by
[@&#8203;chris-ha458](https://togithub.com/chris-ha458) in
[https://github.com/coriolinus/counter-rs/pull/38](https://togithub.com/coriolinus/counter-rs/pull/38)
- deprecate `init` method by
[@&#8203;coriolinus](https://togithub.com/coriolinus) in
[https://github.com/coriolinus/counter-rs/pull/41](https://togithub.com/coriolinus/counter-rs/pull/41)
- do not use deprecated `init` method by
[@&#8203;coriolinus](https://togithub.com/coriolinus) in
[https://github.com/coriolinus/counter-rs/pull/42](https://togithub.com/coriolinus/counter-rs/pull/42)
- With capacity by
[@&#8203;chris-ha458](https://togithub.com/chris-ha458) in
[https://github.com/coriolinus/counter-rs/pull/40](https://togithub.com/coriolinus/counter-rs/pull/40)
- Clippy fixes by
[@&#8203;chris-ha458](https://togithub.com/chris-ha458) in
[https://github.com/coriolinus/counter-rs/pull/43](https://togithub.com/coriolinus/counter-rs/pull/43)
- Implement Serialize and Deserialize by
[@&#8203;matthewmcintire-savantx](https://togithub.com/matthewmcintire-savantx)
in
[https://github.com/coriolinus/counter-rs/pull/46](https://togithub.com/coriolinus/counter-rs/pull/46)

#### New Contributors

- [@&#8203;chris-ha458](https://togithub.com/chris-ha458) made their
first contribution in
[https://github.com/coriolinus/counter-rs/pull/34](https://togithub.com/coriolinus/counter-rs/pull/34)
-
[@&#8203;matthewmcintire-savantx](https://togithub.com/matthewmcintire-savantx)
made their first contribution in
[https://github.com/coriolinus/counter-rs/pull/46](https://togithub.com/coriolinus/counter-rs/pull/46)

**Full Changelog**:
coriolinus/counter-rs@v0.5.7...v0.5.8

</details>

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exercism/rust/parallel-letter-frequency/Cargo.toml

@@ -4,5 +4,5 @@ name = "parallel-letter-frequency"
 version = "0.0.0"
 
 [dependencies]
-counter = "0.5.7"
+counter = "0.6.0"
 rayon = "1.10.0"