Specify minimum Android API level (+ add Animal Sniffer to verify this)

# Problem solved by the feature

The README only specifies that Java 7 is the minimum version for the latest Gson versions. However, for Android developers this information is not that helpful because Java 7 API was added in multiple different Android API levels. For example `java.lang.ReflectiveOperationException` added in Java 7 was only added for Android API level 19 (Android 4.4), see also https://github.com/google/gson/issues/2310#issuecomment-1616848536.



# Feature description

- The README should explicitly state which Android API level is the minimum

- The build setup should verify that no newer Android APIs are used (respectively that desugaring is supported).

This can be verified using Animal Sniffer, see:

  - https://github.com/google/guava/issues/4005

  - Android Scents for Maven

    (to be verified that these artifacts are safe; but appears they are also used by other projects, e.g. OkHttp ([code](https://github.com/square/okhttp/blob/7cd7f9720dc7ff45663c29b7a15a42d27cc845d4/gradle/libs.versions.toml#L82-L83)))

    - https://sourceforge.net/projects/androidscents/

    - https://search.maven.org/search?q=g:net.sf.androidscents.signature

  - Animal Sniffer signatures, considering desugaring: https://github.com/open-toast/gummy-bears

    In that case would have to explicitly mention desugaring (using the latest Android build tools?) in the README



# Alternatives / workarounds

_none_

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