fix warnings

[xuwei-k]

Pull Request Checklist

• Have you read How to write the perfect pull request?
• Have you read through the contributor guidelines?
• Have you referenced any issues you're fixing using commit message keywords?
• Have you added copyright headers to new files?
• Have you checked that both Scala and Java APIs are updated?
• Have you updated the documentation for both Scala and Java sections?
• Have you added tests for any changed functionality?

Helpful things

Fixes

Fix warings

Purpose

Background Context

[warn] /home/runner/work/playframework/playframework/testkit/play-test/src/test/java/play/test/HelpersTest.java:152:1: unchecked conversion
[warn]   required: java.util.Map<java.lang.String,java.lang.String[]>
[warn]   found:    java.util.HashMap
[warn] new java.util.HashMap()

https://github.com/google/guava/blob/4d2b6ffb312a4ebf8aa377982fc98a7ece0bc3fd/android/guava/src/com/google/common/io/Files.java#L428-L429

[warn] /home/runner/work/playframework/playframework/transport/server/play-server/src/test/scala/play/core/server/ProdServerStartSpec.scala:93:23: method createTempDir in class Files is deprecated
[warn]     val temp = GFiles.createTempDir()
[warn]                       ^

References