refactor(lambda): use lowercase boolean for type declarations (#27082)

Updated the type annotations to use the primitive `boolean` type instead of the object wrapper `Boolean`. 
This change aligns with TypeScript's recommended naming conventions and provides clearer type indications.

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Author: yodeee9

packages/aws-cdk-lib/aws-lambda/lib/runtime.ts

@@ -304,7 +304,7 @@ export class Runtime {
   /**
    * Enabled for runtime enums that always target the latest available.
    */
-  public readonly isVariable: Boolean;
+  public readonly isVariable: boolean;
 
   constructor(name: string, family?: RuntimeFamily, props: LambdaRuntimeProps = {}) {
     this.name = name;