2021-December-28 - Prerelease - 2.6.9

NEW: Fixes to errorprone errors (incl. code smells) (#9947)

-Rename Retryable -> Retriable (and RetryableTest -> RetriableTest)
-FileUtilsTest (path with generic line separator)

SuppressWarnings statements for
- UnnecessaryLambda (also turn off completely in game-core/build.gradle)
- SameNameButDifferent (HttpClient.java)

CodeSmell fixes
- ClientSetupPanel.java: empty method without explanation
- EditPanel.java:
1 constants instead of duplicating literal
- HttpClient.java: %n produces platform-specific line separator instead of \n
- SaveGameFileChooser.java: replaceAll is the same as replace when no regex is used, but consumes more performance
- ServerModel.java:
1 use entrySet instead of keySet if both (key and value) are needed
2 Return of empty array (new byte[0]) instead of null