Update dependency io.reactivex.rxjava3:rxjava to v3.1.8 (#915)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [io.reactivex.rxjava3:rxjava](https://togithub.com/ReactiveX/RxJava) |
dependencies | patch | `3.1.7` -> `3.1.8` |

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### Release Notes

<details>
<summary>ReactiveX/RxJava (io.reactivex.rxjava3:rxjava)</summary>

###
[`v3.1.8`](https://togithub.com/ReactiveX/RxJava/releases/tag/v3.1.8)


[Maven](http://search.maven.org/#artifactdetails%7Cio.reactivex.rxjava3%7Crxjava%7C3.1.8%7C)
[JavaDocs](http://reactivex.io/RxJava/3.x/javadoc/3.1.8)

##### Bugfixes

- Fix `ObservableSwitchMap` `NullPointerException` due to cancel race.
([https://github.com/ReactiveX/RxJava/pull/7597](https://togithub.com/ReactiveX/RxJava/pull/7597))

</details>

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gradle/libs.versions.toml

@@ -217,7 +217,7 @@ roborazzi = { module = "io.github.takahirom.roborazzi:roborazzi", version.ref =
 roborazzi-compose = { module = "io.github.takahirom.roborazzi:roborazzi-compose", version.ref = "roborazzi" }
 roborazzi-rules = { module = "io.github.takahirom.roborazzi:roborazzi-junit-rule", version.ref = "roborazzi" }
 
-rxjava = "io.reactivex.rxjava3:rxjava:3.1.7"
+rxjava = "io.reactivex.rxjava3:rxjava:3.1.8"
 
 sqldelight-driver-android = { module = "app.cash.sqldelight:android-driver", version.ref = "sqldelight" }
 sqldelight-coroutines = { module = "app.cash.sqldelight:coroutines-extensions-jvm", version.ref = "sqldelight" }