Gson 2.9.0

The minimum supported Java version changes from 6 to 7.

• Change target Java version to 7 (#2043)
• Put module-info.class into Multi-Release JAR folder (#2013)
• Improve error message when abstract class cannot be constructed (#1814)
• Support EnumMap deserialization (#2071)
• Add LazilyParsedNumber default adapter (#2060)
• Fix JsonReader.hasNext() returning true at end of document (#2061)
• Remove Gradle build support. Build script was outdated and not actively maintained anymore (#2063)
• Add GsonBuilder.disableJdkUnsafe() (#1904)
• Add UPPER_CASE_WITH_UNDERSCORES in FieldNamingPolicy (#2024)
• Fix failing to serialize Collection or Map with inaccessible constructor (#1902)
• Improve TreeTypeAdapter thread-safety (#1976)
• Fix Gson.newJsonWriter ignoring lenient and HTML-safe setting (#1989)
• Delete unused LinkedHashTreeMap (#1992)
• Make default adapters stricter; improve exception messages (#2000)
• Fix FieldNamingPolicy.upperCaseFirstLetter uppercasing non-letter (#2004)

Thanks to our contributors, especially @Marcono1234!