Bump atomicfu to v0.21.0

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
org.jetbrains.kotlinx:atomicfu-gradle-plugin	0.20.2 -> 0.21.0
org.jetbrains.kotlinx:atomicfu	0.20.2 -> 0.21.0

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Release Notes

Kotlin/kotlinx.atomicfu

v0.21.0

Compare Source

• Updated Kotlin to 1.8.20.
• Updated Gradle to 7.3 (#​300).
• Updated kotlinx.metadata version to 0.6.0 (#​281).
• Minimal supported KGP(1.7.0) and Gradle(7.0) versions are set since this release.
• Removed JS Legacy configurations for KGP >= 1.9.0 (#​296).
• Fixed class duplication (from original and transformed directories) in Jar (#​301).
• Original class directories are not modified in case of compiler plugin application (#​312).

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