Add Gradle version compatibility tests

[fzhinkin]

Readme claims that BCV works with Gradle >= 6.0.

There are no tests checking that, so we need to add them.

[fzhinkin]

I tried to add such functional tests, but, for some reason, tests use project's Kotlin version (1.9.22) instead of the version specified in the test's build config. And 1.9.22 requires Gradle 6.8.3 or later.

[aSemy]

for some reason, tests use project's Kotlin version (1.9.22) instead of the version specified in the test's build config

This is probably because of withPluginClasspath()

binary-compatibility-validator/src/functionalTest/kotlin/kotlinx/validation/api/TestDsl.kt

Line 40 in 3a70033

.withPluginClasspath()