Fix tests under Java 17 (Guice upgrade via Play)

After #93 we found that
unfortunately the Release workflow failed while running the tests:

* The Release workflow uses Java 17 for all builds
* The Atom Maker library uses Play with Guice for Dependency Injection
* Guice gained Java 17 & 21 support with Guice v6:
  https://github.com/google/guice/wiki/Guice600
* Play only updated to Guice v6 (and gained general Java 17 support)
  with Play v2.9:
  playframework/playframework#11808
  playframework/playframework@10ca54d#diff-3dc52110c1c1c453c2e9740ac58fe7e90d53121875a034ef3109c34ab030c29e

atom-manager-play-lib/build.sbt

@@ -5,7 +5,7 @@ name := "atom-manager-play"
 libraryDependencies ++= Seq(
   "com.typesafe.play"      %% "play"                  % playVersion,
   "com.gu"                 %% "content-atom-model"    % contentAtomVersion,
-  "org.scalatestplus.play" %% "scalatestplus-play"    % "5.1.0" % Test,
+  "org.scalatestplus.play" %% "scalatestplus-play"    % "6.0.1" % Test,
   "com.amazonaws"          %  "aws-java-sdk-dynamodb" % awsVersion,
   "org.mockito"            %  "mockito-core"          % mockitoVersion % Test,
   "com.typesafe.play"      %% "play-test"             % playVersion % Test,

build.sbt

@@ -6,8 +6,8 @@ name := "atom-maker-lib"
 
 lazy val artifactProductionSettings = Seq(
   organization := "com.gu",
-  scalaVersion := "2.12.18",
-  crossScalaVersions := Seq(scalaVersion.value, "2.13.12"),
+  scalaVersion := "2.13.12",
+  crossScalaVersions := Seq(scalaVersion.value),
   licenses := Seq(License.Apache2),
   scalacOptions := Seq("-deprecation", "-feature", "-release:8")
 )

project/BuildVars.scala

@@ -4,6 +4,6 @@ object BuildVars {
   lazy val awsVersion         = "1.11.8"
   lazy val contentAtomVersion = "4.0.0"
   lazy val scroogeVersion     = "22.1.0"
-  lazy val playVersion        = "2.8.8"
-  lazy val mockitoVersion     = "4.8.0"
+  lazy val playVersion        = "2.9.1"
+  lazy val mockitoVersion     = "4.11.0"
 }

project/plugins.sbt

@@ -1,4 +1,4 @@
-addSbtPlugin("com.typesafe.play" % "sbt-plugin" % "2.8.8")
+addSbtPlugin("com.typesafe.play" % "sbt-plugin" % "2.9.1")
 
 addSbtPlugin("com.github.sbt" % "sbt-release" % "1.1.0")
 addSbtPlugin("org.xerial.sbt" % "sbt-sonatype" % "3.10.0")