Update dependency com.google.code.gson:gson to v2.9.0

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
com.google.code.gson:gson	2.8.9 -> 2.9.0

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Release Notes

google/gson

v2.9.0

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 Build script was outdated and not actively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)

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