[BEAM-4328] Upgrade Gradle shadowJar plugin to latest.

[swegner]

http://imperceptiblethoughts.com/shadow/#change_log

This is an attempt to fix release build issues described in BEAM-4328. I don't have any confidence that it fixes the build. But it might, and it's general goodness to keep our dependencies up-to-date.

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[swegner]

R: @pabloem

CC: @boyuanzz @lukecwik

[pabloem]

LGTM. I'll merge after getting passing precommit.

[swegner]

Java pre-commits fail on task :beam-sdks-java-maven-archetypes-starter:generateAndBuildArchetypeTest: https://builds.apache.org/job/beam_PreCommit_Java_GradleBuild/5505/

@pabloem points out that this also failed on another PR which doesn't change Java: https://scans.gradle.com/s/kr23q43mh6fmk

[swegner]

Run Java PreCommit

[pabloem]

Nice. Merging.