fix(deps): update dependency io.reactivex.rxjava3:rxjava to v3.1.8 (#168)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [io.reactivex.rxjava3:rxjava](https://togithub.com/ReactiveX/RxJava) |
`3.1.7` -> `3.1.8` |
[![age](https://developer.mend.io/api/mc/badges/age/maven/io.reactivex.rxjava3:rxjava/3.1.8?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![adoption](https://developer.mend.io/api/mc/badges/adoption/maven/io.reactivex.rxjava3:rxjava/3.1.8?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
[![passing](https://developer.mend.io/api/mc/badges/compatibility/maven/io.reactivex.rxjava3:rxjava/3.1.7/3.1.8?slim=true)](https://docs.renovatebot.com/merge-confidence/)
|
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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

@@ -50,7 +50,7 @@ okhttp = "4.12.0"
 retrofit = "2.9.0"
 robolectric = "4.11.1"
 rxandroid3 = "3.0.2"
-rxjava3 = "3.1.7"
+rxjava3 = "3.1.8"
 rxkotlin3 = "3.0.1"
 semver = "1.4.2"
 sparkbutton = "4.1.0"