Update plugin nebula.release to v20.2.0

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
nebula.release	20.0.0 -> 20.2.0

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Configuration

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[breedx-splk]

oh sure

[trask]

oh sure

you would think... and yet 😭

FAILURE: Build failed with an exception.

* Where:
Build file '/home/runner/work/opentelemetry-proto-java/opentelemetry-proto-java/build.gradle.kts' line: 77

* What went wrong:
Script compilation error:

  Line 77:   NearestVersionLocator(release.gitReadCommands, TagStrategy()).locate().any.toString()
                                           ^ Unresolved reference: gitReadCommands

[trask]

I don't love nebula anyways, so I probably will replace with explicit versioning similar to the other Java repos