[Kotlin][Spring] Prevent double-quoting default values #9904

Created by: pilzm

Looks like the change in #8577 fixed default values for Java generators, but caused kotlin-spring to break with #9904. The kotlin generators handle default values in their own way in org.openapitools.codegen.languages.AbstractKotlinCodegen#toDefaultValue, so string values end up quoted twice (e.g. defaultValue = ""id"").

For this fix, I just reverted the relevant change to kotlin-spring in #8577. I assume this is a non-breaking change - non-string values should still be quoted like before.

To test this fix: I verified this fix with the reproducer provided in #9904, and the resulting diff is empty -> original behavior restored.

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