deps: update dependency gcp-releasetool to v1.8.8 (#764)

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Author: renovate-bot

README.md

@@ -57,13 +57,13 @@ implementation 'com.google.cloud:google-cloud-texttospeech'
 If you are using Gradle without BOM, add this to your dependencies:
 
 ```Groovy
-implementation 'com.google.cloud:google-cloud-texttospeech:2.4.4'
+implementation 'com.google.cloud:google-cloud-texttospeech:2.4.6'
 ```
 
 If you are using SBT, add this to your dependencies:
 
 ```Scala
-libraryDependencies += "com.google.cloud" % "google-cloud-texttospeech" % "2.4.4"
+libraryDependencies += "com.google.cloud" % "google-cloud-texttospeech" % "2.4.6"
 ```
 
 ## Authentication