Bump retrofit-version from 2.3.0 to 2.8.1

Bumps `retrofit-version` from 2.3.0 to 2.8.1.

Updates `converter-gson` from 2.3.0 to 2.8.1

Updates `retrofit` from 2.3.0 to 2.8.1
- [Release notes](https://github.com/square/retrofit/releases)
- [Changelog](https://github.com/square/retrofit/blob/master/CHANGELOG.md)
- [Commits](square/retrofit@parent-2.3.0...parent-2.8.1)

Updates `converter-scalars` from 2.3.0 to 2.8.1

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

pom.xml

@@ -215,7 +215,7 @@
         <maven.compiler.target>${java.version}</maven.compiler.target>
         <okhttp-version>2.7.5</okhttp-version>
         <gson-fire-version>1.8.0</gson-fire-version>
-        <retrofit-version>2.3.0</retrofit-version>
+        <retrofit-version>2.8.1</retrofit-version>
         <threetenbp-version>1.3.5</threetenbp-version>
         <junit-version>4.12</junit-version>
     </properties>