[kotlin-spring] fix BigDecimal default value failed to compile

[irasychan]

Fixes #14584

When a property is of type number and has a default value, the Kotlin Spring generator will generate code like val bar: java.math.BigDecimal? = 0, which will not compile, since an Int cannot be assigned to BigDecimal. This PR fixes that bug.

This PR applies changes from #12213 to kotlin-client to kotlin-spring

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@jimschubert @dr4ke616 @karismann @Zomzog @andrewemery @4brunu @yutaka0m @stefankoppier

[irasychan]

As the reviewer assigned, @jimschubert, is no longer active in this repository, according to his resume site and his Github activity over the past 2 years.

@wing328 @4brunu @stefankoppier
Would you be able to help with this PR?

[4brunu]

Looks good to me 👍