Update dependency io.reactivex.rxjava3:rxjava to v3.1.8 (#662)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [io.reactivex.rxjava3:rxjava](https://github.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://github.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://github.com/ReactiveX/RxJava/pull/7597))

</details>

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gradle/libs.versions.toml

@@ -94,5 +94,5 @@ okio-fakefilesystem = { module = "com.squareup.okio:okio-fakefilesystem", versio
 oshi = "com.github.oshi:oshi-core:6.4.8"
 retrofit = { module = "com.squareup.retrofit2:retrofit", version.ref = "retrofit" }
 retrofit-converters-wire = { module = "com.squareup.retrofit2:converter-wire", version.ref = "retrofit" }
-rxjava = "io.reactivex.rxjava3:rxjava:3.1.7"
+rxjava = "io.reactivex.rxjava3:rxjava:3.1.8"
 truth = "com.google.truth:truth:1.1.5"