Bump to Checker dataflow 3.20.0

[msridhar]

The key new feature here is that the dataflow library no longer crashes on switch expressions / arrow case labels in switch statements:

https://github.com/typetools/checker-framework/blob/checker-framework-3.20.0/docs/CHANGELOG.md#version-3200-december-6-2021

This is an initial step toward fixing #289, though we will need precise control-flow graphs (which is still a WIP in Checker dataflow) to really handle these constructs properly.

[coveralls]

Pull Request Test Coverage Report for Build #590

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 89.402%

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Totals
Change from base Build #589:	0.0%
Covered Lines:	3602
Relevant Lines:	4029

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💛 - Coveralls

[msridhar]

JMH Results. Before:

Benchmark                      Mode  Cnt   Score   Error  Units
AutodisposeBenchmark.compile  thrpt   25  17.281 ± 0.092  ops/s
CaffeineBenchmark.compile     thrpt   25   4.253 ± 0.028  ops/s

After:

Benchmark                      Mode  Cnt   Score   Error  Units
AutodisposeBenchmark.compile  thrpt   25  17.439 ± 0.088  ops/s
CaffeineBenchmark.compile     thrpt   25   4.255 ± 0.017  ops/s

[lazaroclapp]

LGTM!