Bump retrofitVersion from 2.3.0 to 2.9.0

Bumps `retrofitVersion` from 2.3.0 to 2.9.0.

Updates `retrofit` from 2.3.0 to 2.9.0
- [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...2.9.0)

Updates `converter-gson` from 2.3.0 to 2.9.0
- [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...2.9.0)

Updates `retrofit-mock` from 2.3.0 to 2.9.0
- [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...2.9.0)

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

build.gradle

@@ -3,7 +3,7 @@ buildscript {
         kotlinVersion = "1.4.31"
         // MOBILE-3449 Parameters encoding changed in later versions https://github.com/square/okhttp/issues/3235
         okHttpVersion = "3.9.1"
-        retrofitVersion = "2.3.0"
+        retrofitVersion = "2.9.0"
         spekVersion = "2.0.15"
     }
     repositories {