Update dependency com.google.code.gson:gson to v2.9.1

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
com.google.code.gson:gson	2.9.0 -> 2.9.1

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Release Notes

google/gson

v2.9.1

• Make Object and JsonElement deserialization iterative rather than
  recursive (#​1912)
• Added parsing support for enum that has overridden toString() method (#​1950)
• Removed support for building Gson with Gradle (#​2081)
• Removed obsolete codegen hierarchy (#​2099)
• Add support for reflection access filter (#​1905)
• Improve TypeToken creation validation (#​2072)
• Add explicit support for float in JsonWriter (#​2130, #​2132)
• Fail when parsing invalid local date (#​2134)

Also many small improvements to javadoc.

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