Bump up spotless-lib to 2.20.0

[KengoTODA]

Hello 👋 thanks for sharing such a nice Gradle plugin!
It helps a lot to remove dependency on Eclipse in local.

I'm trying to introduce this plugin to a Gradle project depending on the spotless gradle plugin.
But when I run the build, it failed due to the following error:

Receiver class com.diffplug.gradle.spotless.GradleProvisioner$DedupingProvisioner does not define or inherit an implementation of the resolved method 'abstract java.util.Set provisionWithDependencies(java.util.Collection)' of interface com.diffplug.spotless.Provisioner.

I found that this plugin depends on old spotless-lib, and it brings a conflict into the classpath. So in this PR, I want to suggest upgrading the spotless-lib to 2.20.0.

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Please make sure that your PR allows edits from maintainers. Sometimes its faster for us to just fix something than it is to describe how to fix it.

After creating the PR, please add a commit that adds a bullet-point under the -SNAPSHOT section of CHANGES.md that includes:

• a summary of the change
• a link to the newly created PR

This makes it easier for the maintainers to quickly release your changes.

[KengoTODA]

note: I use spotless-lib:2.20.0 which is not latest but used from the gradle/6.0.0.
I'm motivated to use this version, to avoid build failure caused by new API in the gradle/6.0.1 diffplug/spotless#872.

[nedtwigg]

Published in 3.34.1