Release 3.20.0 of the Checker Framework

[Suzanne Millstein]

We have released a new version of the Checker Framework.
The Checker Framework lets you create and/or run pluggable type checkers, in order to detect and prevent bugs in your code.

You can find documentation and download links at:
http://CheckerFramework.org/

Changes for Checker Framework version 3.20.0:

**User-visible changes:**

The Checker Framework now runs on code that contains switch expressions and switch statements that use the new `->` case syntax, but treats them conservatively. A future version will improve precision.

**Implementation details:**

The dataflow framework can be run on code that contains switch expressions and switch statements that use the new `->` case syntax, but it does not yet analyze the cases in a switch expression and it treats `->` as `:`. A future version will do so.

Removed methods and classes that have been deprecated for more than one year:
 * Old way of constructing qualifier hierarchies
 * `@SuppressWarningsKeys`
 * `RegularBlock.getContents()`
 * `TestUtilities.testBooleanProperty()`
 * `CFAbstractTransfer.getValueWithSameAnnotations()`

**Closed issues:**
#4911, #4948, #4965.