Consider updating JDK version for java-json-tools repos

[Capstan]

JDK 1.6 is hard to build on these days:

• Gradle 5.6.1 marks it deprecated and to be removed.
• Xcode 10.3 (10G8) on Mojave doesn't include it, making it difficult to do development.
• Guava 21 and up require JDK 1.8, and due to CVE-2018-10237, we're required to use at least Guava 25. This might be rendered moot if we address Consider removal of Guava from java-json-tools repos #11.

We should consider upgrading the JDK, presumably to JDK 1.8.

Things to consider:

• Do we bump the major version number given that the source compatibility would change? JDK 1.6 Compatibility guide
• Do we want to target JDK 1.7 for Android support? I know very little about this save that https://github.com/google/guava has different dependencies for Android vs. the JRE, and right now we're using the JRE (which might interfere with our Android clients; don't know the details).

/cc @huggsboson

[Capstan]

Asked a question in google/guava#3595 about how downstream projects are expected to provide JRE vs. Android support if they're Guava consumers.

[Capstan]

Forgot these docs too:

• Compatibility Guide for JDK 8
• Java SE 7 and JDK 7 Compatibility

[Capstan]

Per advice on google/guava#3595, posted a question on StackOverflow.

[cpovirk]

• Do we bump the major version number given that the source compatibility would change?

I don't know whether there's a settled answer to this, so I am replying half to link to this Google Cloud proposed best practice, which suggests not bumping, and half to subscribe to this thread to hear if other people have recommendations to the contrary :)

[Capstan]

Given @cpovirk's advice, I propose:

• We switch the reference to Guava from the JRE-flavor to the Android-flavor (and people can use it with the JRE anyway).
• We update the source/target compatibility to 1.7, since it's enforced by our use of Guava.
• We only bump the minor release # instead of the major release #, because 1.6 is essentially end-of-life.
• We remove any uses of deprecated and removed symbols so that we can upgrade all the java-json-tools to 28.1-android, rather than being unfortunately pinned back at 25.

Feedback accepted until end of September, after which point I'll make the requisite changes.