Update zio-streams, zio-test-sbt to 2.1.8 (#1316)

## About this PR
📦 Updates 
* [dev.zio:zio-streams](https://github.com/zio/zio)
* [dev.zio:zio-test-sbt](https://github.com/zio/zio)

 from `2.1.7` to `2.1.8`

📜 [GitHub Release Notes](https://github.com/zio/zio/releases/tag/v2.1.8)
- [Version Diff](zio/zio@v2.1.7...v2.1.8)

## Usage
✅ **Please merge!**

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don't change it yourself.

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<details>
<summary>⚙ Adjust future updates</summary>

Add this to your `.scala-steward.conf` file to ignore future updates of
this dependency:
```
updates.ignore = [ { groupId = "dev.zio" } ]
```
Or, add this to slow down future updates of this dependency:
```
dependencyOverrides = [{
  pullRequests = { frequency = "30 days" },
  dependency = { groupId = "dev.zio" }
}]
```
</details>

<sup>
labels: library-update, early-semver-patch, semver-spec-patch,
commit-count:1
</sup>

Co-authored-by: zio-scala-steward[bot] <145262613+zio-scala-steward[bot]@users.noreply.github.com>

build.sbt

@@ -25,7 +25,7 @@ lazy val _scala3   = "3.3.3"
 inThisBuild(
   List(
     name         := "ZIO Kafka",
-    zioVersion   := "2.1.7",
+    zioVersion   := "2.1.8",
     scalaVersion := _scala213,
     // zio-sbt defines these 'scala213' and 'scala3' settings, but we need to define them here to override the defaults and better control them
     scala213 := _scala213,