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

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This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
com.google.code.gson:gson	2.8.6 -> 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 Build script was outdated and not actively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)

v2.8.9

• Make OSGi bundle's dependency on sun.misc optional (#​1993).
• Deprecate Gson.excluder() exposing internal Excluder class (#​1986).
• Prevent Java deserialization of internal classes (#​1991).
• Improve number strategy implementation (#​1987).
• Fix LongSerializationPolicy null handling being inconsistent with Gson (#​1990).
• Support arbitrary Number implementation for Object and Number deserialization (#​1290).
• Bump proguard-maven-plugin from 2.4.0 to 2.5.1 (#​1980).
• Don't exclude static local classes (#​1969).
• Fix RuntimeTypeAdapterFactory depending on internal Streams class (#​1959).
• Improve Maven build (#​1964).
• Make dependency on java.sql optional (#​1707).

v2.8.8

• Fixed issue with recursive types (#​1390).
• Better behaviour with Java 9+ and Unsafe if there is a security manager (#​1712).
• EnumTypeAdapter now works better when ProGuard has obfuscated enum fields (#​1495).

v2.8.7

• Fixed ISO8601UtilsTest failing on systems with UTC+X.
• Improved javadoc for JsonStreamParser.
• Updated proguard.cfg (#​1693).
• Fixed IllegalStateException in JsonTreeWriter (#​1592).
• Added JsonArray.isEmpty() (#​1640).
• Added new test cases (#​1638).
• Fixed OSGi metadata generation to work on JavaSE < 9 (#​1603).

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