added JDK8 compatibility for smithy-ai-traits

[yasmewad]

We want to be able to consume the traits package in JDK 8 packages too. Fixed JDK 8 compatibility in validator tests.

Testing:
./gradlew clean build

❯ javap -v 'jar:file:////Volumes/workplace/gateway-live/src/smithy-java/smithy-ai-traits/build/libs/smithy-ai-traits-0.0.2.jar!/software/amazon/smithy/ai/PromptsTrait.class' 'jar:file:////Volumes/workplace/gateway-live/src/smithy-java/smithy-ai-traits/build/libs/smithy-ai-traits-0.0.2.jar!/software/amazon/smithy/ai/PromptTemplateDefinition.class' | grep -B 2 "version:"
  Compiled from "PromptsTrait.java"
public final class software.amazon.smithy.ai.PromptsTrait extends software.amazon.smithy.model.traits.AbstractTrait implements software.amazon.smithy.utils.ToSmithyBuilder<software.amazon.smithy.ai.PromptsTrait>
  minor version: 0
  major version: 52
--
  Compiled from "PromptTemplateDefinition.java"
public final class software.amazon.smithy.ai.PromptTemplateDefinition extends java.lang.Object implements software.amazon.smithy.model.node.ToNode, software.amazon.smithy.utils.ToSmithyBuilder<software.amazon.smithy.ai.PromptTemplateDefinition>
  minor version: 0
  major version: 52

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