Update testcontainers-java monorepo to v1.20.6

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
org.testcontainers:junit-jupiter (source)	1.20.5 -> 1.20.6
org.testcontainers:testcontainers (source)	1.20.5 -> 1.20.6
org.testcontainers:mockserver (source)	1.20.5 -> 1.20.6

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Release Notes

testcontainers/testcontainers-java (org.testcontainers:junit-jupiter)

v1.20.6

Compare Source

What's Changed

• Bump confluentinc/cp-kcat from 7.4.1 to 7.9.0 (#​10000) @​julianladisch
• Set sourceCompatibility and targetCompatibility to 1.8 in spock module

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