Update dependency org.apache.maven.plugins:maven-jar-plugin to v3

[renovate-bot]

This PR contains the following updates:

Package	Update	Change
org.apache.maven.plugins:maven-jar-plugin	major	2.5 -> 3.1.2

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[codecov]

Codecov Report

Merging #6020 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #6020 +/- ##
============================================
- Coverage 47.35% 47.35% -0.01% - Complexity 27110 27125 +15 
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Files 2516 2516 Lines 273611 273611 Branches 31353 31348 -5 ============================================
- Hits 129566 129565 -1 - Misses 134436 134437 +1 
Partials 9609 9609

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/com/google/cloud/pubsub/v1/Publisher.java	89% <0%> (-0.35%)	40% <0%> (ø)

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