Update dokka to v1.8.20

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
org.jetbrains.dokka:dokka-core	1.8.10 -> 1.8.20
org.jetbrains.dokka:dokka-gradle-plugin	1.8.10 -> 1.8.20

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