Apply implicit conversion from derived Conversion instance defined as implicit rather than given

[prolativ]

Fixes #21757

[prolativ]

This seems to fix the problem, assuming usage of Conversion instances for implicit conversions is expected to work if the instance is defined as implicit def rather than given.
The specification https://docs.scala-lang.org/scala3/reference/contextual/conversions.html mentions only given instances but I assume this was intended to take implicit into account.
Anyway, this already works with implicit for simpler cases